I have both footflex and prostretch and find the prostretch superior.
I, like some others, find the footflex strains the muscle running from the big toe under the ball of the foot(fdb). This can be relieved by peeking your big toe out to the side so that it is not stretched as agressively as the other toes (fdl). Others do not report this problem and are happy with it. In my opinion, if you wear shoes which flex at the toes (as you should!) then you can just put your toes on a book or something and it will do the same as the footflex.
The prostretch gives a great calf stretch with controlled resistance superior to a stair-stretch and is worth the money in my opinion. While the prostretch works with some weight bearing, it is controllable with your other planted foot. weight bearing is part of life and so every pf sufferer should go to weight bearing stretches eventually. Depending on the case, it might not be advisable to do much or any weight bearing stretches in the early stages, but this is a minority opinion voiced only by me and other inventors who have developed non-weight bearing stretching devices, so a grain of salt is in order.