This article is written by an actual amputee who wears these items. There are quite a few things to consider in selecting a prosthetic knee.
Certainly the most well known knee on the market is the C-Leg from Otto Bock, but it may not be the best for you because if your insurance will not pay for all it's needs of service, you'll find yourself dipping into your pocket uncontrollably. This article will only address the three most popular knees, the C-leg, the Rheo Knee and the Adaptive - or Smart Adaptive as it is known these days.
People throw words around today without thought as to what they really mean.
One web definition of "bionic" is "Having anatomical structures or physiological processes that are replaced or enhanced by electronic or mechanical components." (http://www.thefreedictionary.com/bionic)
Some prosthetic knees are not worth considering. For instance a "safety knee" is just as dangerous as it is inexpensive.
A "four bar knee" is really reliable, and bends close to the socket being good for a long stump; however, whether hydraulic like the Century 1900, 2000 and 2100, or pneumatic like the Teh-Lin (DAW-USA) they cannot take a lot of walking. They get hot and loose the resistance required for dampening in the swing phase.
People with two good legs - and even Below Knee amputees cannot understand . . . without a knee you can control with muscles, you are at the mercy of your eyes, your balance and the level or smoothness of the terrain.
All amputees are not amputated equal, and you are not a REAL amputee unless you are an Above Knee amputee. Hip dis-articulated amputees suffer an even worse hazard. Even the State of California does not consider you "disabled" for the purposes of a Hunting or Fishing license UNLESS you are amputated ABOVE the knee.
All prosthetic knees are not created equal, and you have to WALK on them to know the difference. That puts "two legged people" out in the cold as being able to make decisions or tell an above knee amputee ANYTHING about what "might" work best for them.
If you want a fairly good and safe knee - you might want to try the Geoflex knee from Ohio Willow Wood Company.
I owned one of these knees and walked on it comfortably and with the most secure safety I have felt on any knee I have owned.
The only problem with this knee is it uses a rubber return strap, and it tends to shear the pin at the front of the knee where the Flexion functions on the nylon insert to the rear. See the pic.