I apologize for the delay!!
I'm not good with input on this as my 'equipment' is extremely outdated. Any issues I have may very well be because I just need a new prosthesis. Therefore, I do not want to say anything that would steer you away from a Symes amputation.
What I will say: I know some Symes can put weight on the end of their leg (meaning, walk to the bathroom in the middle of the night) without putting on their prosthesis. I am not one of those. However, my amputation was done in 1978. They (meaning the surgeons) may have done things differently back then than they do now. (?) No clue.
While at the ACA conference I heard of a Symes who runs marathons (much to my astonishment and awe)! I want to email him but can't recall his name.
The advice to talk with your prosthetist and surgeon to get a list of pros-n-cons to each procedure sounds like the best advice. In addition, if you go with the B/K, you'll want to figure out with them how high to go to maintain the best leverage yet also be able to purchase the components you want to have the life that you want.
You've got A LOT of research a head of you.