Orisson,
You didn't mention they type of prosthetics you're using and that's important. Most athletes have hydraulic knees which are "single axis" knees. There are also some "multi-axis" hydraulic knees out there. These knees are supposed to better mimic the action a god-given leg makes. These knees can also work out real well for the above knee amputee who wants to engage in sports, such as walking/hiking.
I work with athletes with disabilities and have found that most above the knee amputees can manage a 20 lb pack just fine with the right prosthetics.
Here's the reasoning...the weight is balanced over your legs, thus the pressures are similar to normal exertion. If you go with a cart, you're now leaning forward and rather than balancing on your legs...you're pushing into your prosthetic. If you don't have hydraulics, but rather a cup, as the stump rides in the cup it tends to cause friction and added wear that can be quite painful.
Buen Camino,
Arn