Otto,
I think you have a medical conflict going on here, Propecia WILL interupt the effects of Testosterone suppliments, I have a very hard time believing a Doctor prescribes both at the same time, I am around the gym quite often myself, I would get a better medical opinion on this. Propecia directly kills T-cycles and the effectiveness.
If you chose Testosterone injections you vote against keeping most of your hair, but if you have to have them (understandably so) you may (in time)have a sparsly transplanted head of hair, but feel better.
IMO, you are offsetting each drug and wasting $$$$.
All the best with your choices !