Knee replacement is so much better than arthroscopy knee surgery according to my wife. She hurt for 12 months after her orthoscopy, 3 months on the knee replacement. (2 weeks after surgery whe was down here house hunting with me, admittedly on drugs but still she was mobile) The only reason a TKR should hurt you worse is if the doctor screws up the alignment (Always check out your doctor in the area) or puts the wrong joint in, or you don't do all of your therapy afterwords. I don't know of any reason why having a TK would limit you to only flat ground. You should have more mobility after a TKR than you do before. The real limiting factor would be the Quad muscles, that is a problem for my wife because she could never develop hers with her bad real knees. The new knees are supposed to have replaceable inserts on them now so as long as you never go metal to metal in the join itself that should be an easy procedure for you in 10 to 15 years (if you need it).
Now one thing here doctor recommended (she had both done) is to stagger the replacements (i.e. don't do them both at the same time) It is harder to accomplish therapy with two recovering knees and no strength to move around, it increases time on the table and increases your infection possibilities on the table since the procedure usually takes 2 to 3 hours for one knee.