Bi-Fins Guide:

Fin choice:
Apart from mask choice / selection, I believe that fin choice is the next-most important decision that a diver will make about their equipment from an enjoyment and comfort standpoint (assuming you are warm enough with the proper wetsuit chosen).

Fins are our key into the underwater universe that allows us to instantly become aquatic, fast, efficient and graceful.  They also play a key role in the safety of ourselves and for those that we are diving with.

Freediving / apnea / spearfishing fins (bi-fins): 
These are significantly longer that scuba, snorkelling or boogie boarding fins for increased efficiency and are typically anywhere from 80cm - 105cm with most being around 90-95cm.

Footpocket:
This is the connection point between the diver and the fin.  Proper fitment is critical for comfort and to allow for optimal power transfer / efficiency.

Foot shape is a significant consideration, as those with wide feet will not be comfortable in a narrow footpocket, etc.

Footpocket tendon stiffness is important as well.  Too stiff and the blade is a solid plank until the tendon ends and the only flex happens in the bottom half of the blade, decreasing efficiency and increasing effective blade stiffness & effort required.  Too soft and the tendon does not support the blade adequately and too much load can be transferred to the top portion of the blade and over-stressing it, sometimes to the point of breakage / failure (major safety issue).

Power transfer of a particular footpocket (assuming a good fit) is related to the build / design of the footpocet chassis itself.  A stiffer bottom of the footpocket will prevent it from wanting to fold / collapse under load and will help to reduce foot pressure and cramping.  Some of the better engineered foot pockets use a variety of rubber durometers (stiffnesses) for various regions of the footpocket for desired characteristics.  A softer upper area around your Achilles tendon will improve comfort, whereas a firm section above the ball of the foot will assist in power transfer.

When considering fit, one must consider what thinkness (if any) of sock you will be wearing with the fins, based on your water temperatures.  In water less than 12*C surface temp, I like to wear a 5-6mm sock so that my extremities stay warm and circulating blood for 5* water or those under-ice dives mid-winter.  If you try to jam your 5mm socks into fins that you normally wear barefoot, you will end up cutting off your circulation and will be self-defeating.

I have one pair for 6mm+ socks and another for 2mm booties.  Any sock over 3mm, and the power transfer really deteriorates as the neoprene is squishing around and not driving force into the blade.  You can get firm 2mm and 3mm neoprene socks that still allow for decent power transfer.

Blade stiffness:
This is a difficult topic, because the ratings of various manufacturers are not equal.  In fact, it is a rather subjective measurement and many are considered: Soft, Medium, Stiff, Ex-Stiff.  Some have also added a numerical scale (20, 25, 30, 35, etc.)  Others use 1, 2, 3, 4.

As a general rule, you do not want a stiff fin, as you will need more muscular expenditure (and corresponding O2 consumption that goes along with it) to flex a very stiff fin.  Too soft, and the fin bends too much and little thrust is produced.  This is why the most common fin stiffness used by freedivers is "medium"... whatever that means.  It's fin-stiffness selection a la Goldilocks.

Having said that... if the fin manufacturer has done their research... this isn't a bad place to start... especially if you are of average size / strength and dive in moderate depths.

If you are a 260# man, with a football player body-type, who spearfishes in a 7mm suit in 20m+ depth that wears 20 lbs of lead... a medium stiffness fin might be REALLY scary when trying to ascend from the bottom while fighting a fish, due to the massive buoyancy change of a 7mm suit at 20m depth with that much lead.

If you are larger and a more aggressive or stronger kicker, something slightly stiffer might be worth considering.  If you dive DEEP (50m+), a soft fin with a 5mm suit might not inspire confidence as you start your way up as it bends under you like a wet towel.

Having said all of that... I believe that William Winram who has dived deeper than anyone with bi-fins (94m) did so with medium stiffness fins.  Good technique (power coming from the hip flexors and minimizing knee bend) and rhythm can allow a diver to use a softer fin than someone with poor "bicycling" technique.

Blade material:
There are 3 common materials for apnea fins:

• Plastic: cheap, durable, heavy somewhat muted recoil back to straight
• Fibreglass Composite: medium cost, more responsive and lighter than plastic, more fragile than plastic
• Carbon: most expensive, thin blade for incredible lightness, most responsiveness / highest performance, excellent recoil for added thrust efficiency, more fragile than plastic

Note:  MANY blades that are advertised as carbon or LOOK like carbon fibre are actually a mix of FG and Carbon and only have a veneer (1-2 layers) of carbon fibre on both sides of a primarily fibreglass blade. The properties are generally closer to FG, with a slightly improved snap-back due to the carbon.

Angle:
Blade angle is the difference between the bottom of the foot and the angle of the blade. An angled blade makes surface swimming easier than one without an angle.  As a general rule of thumb, somewhere between 20* and 25* is a good compromise and allows for decent power on the up-kick as well.

Weight:
The lighter the fin, the less effort / energy is expended moving the equipment itself.  Lighter is better, provided that structural compromises have not been made.  With a heavy fin (like the Mares Avanti Quattro rubber fin), your legs get very tired from the constant effort of moving that weight around at the end of the lever / your legs.  This is where a carbon fin and lightweight footpocket combo really shines!

Works as a total "system":
What is important, is that the footpocket and blade fits you personally (both physical fitment as well as stiffness and diving personality) and works together as a system.
EG: a great footpocket like the Omer Stingray (with stiffer tendons) when combined with a long, soft FG blade would result in a fin with only 1/2 of the length bending and producing sub-optimal thrust.

Apneaddict's personal fin choice:
For general all-around use (line diving training, recreational diving, spearfishing, etc.) in all but the coldest of conditions, I use Fins4U.com 'Pure Evolution' 100% Carbon fins in Medium stiffness, paired with the amazing Pathos footpocket.

The 'Pure Evolution' is a 100% pure carbon blade that is extremely responsive and light.  Fins4U.com does carry Leaderfins fibreglass and FG / Carbon mix fins, but has created their own 'pure' carbon fin with a premium footpocket for uncompromising performance.  Make no mistake - there is nothing wrong with the Leaderfins lineup - especially when ordered through Fins4U.com with the 22* Pathos footpocket, but the 'Pure Evolution' really is the cream of the crop.

The medium stiffness is a little bit stiffer than most "mediums", but not quite halfway to a "stiff" fin.  This is perfect for me, as I am a strong 80kg kicker that dives beyond 40m and wants some extra strength for wrestling a fish up from depth or in case I need to help to surface a buddy who has blacked out.

I think that for a lighter diver that does not have extremely strong legs, perhaps a "soft" version would suit them better.  

When ascending from depth, the fins feel great when using calm, smooth, wide kicks.  When pushed with extra force or with an aggressive dolphin kick, they really provide some extra snap and rocket you up to the surface so quickly you'll be giddy with laughter.

The Pathos Fireblade footpocket is perhaps the best footpocket on the market and is widely regarded as such by those who have tried several and it is undoubtedly the lightest by a long shot!

Lightweight does not equal weak however.  The Pathos Fireblade uses a variety of rubber compounds to optimize it for power delivery while also retaining amazing comfort. The pocket is then glued directly onto the blade for a permanent bond.  This is stronger than a more conventional drilling and fastening system, which by design creates a weakness by drilling.  Note the backwards "6" on the mold.  :D

There is a stiffer reinforcement panel on the top of the footpocket, just below the "Fireblade" wording that is stiffer than that nearest the toes and Achilles areas.  This panel REALLY helps in creating a solid feeling footpocket with the best power transfer that I have ever felt.  An amazing feat for such a light pocket!

Some people with high arches find this area too tight and there are a couple of solutions if you still want to use the Pathos footpocket (and you SHOULD!).  One suggestion is to shove a couple of D-cell batteries, lead weight or other packing material (newspaper, etc.) into the front of the footpocket and let it stretch out a bit.  Another option is to wear your intended neoprene sock (or cotton / wool one if you intend to go barefoot) and use a hair dryer on high to heat the pocket and then cool / mold back around your feet.  You will get distinct left and right fins after this procedure.

Another option is to opt for the "soft" Pathos footpocket of the same size, as it is a little more compliant and might not be as tight on you... but I would not suggest this if you are using a fin on the upper half of the stiffness scale, as the tendons may not provide enough support on hard / heavy kicks.  Likely a non-issue with soft blades.

I have fairly wide feet near the ball of my foot and I have no issues fitting into these pockets.

For cold water and for diving under ice:
I have a pair of Immersion e:Green plastic fins that can fit up to a 7mm sock for COLD water.  Note that because they are plastic, they really stiffen up below about 6* and can get quite hard.  They are cheap and keep my feet warm in these uncomfortable conditions.  Click to get my cold-water fin modification tips:

Under the ice in a quarry in Quebec in Fenruary: