Alright, did some research, the 650ti is much better than the R7 250, the R7 is basically a piece of crap...
But 650ti compared to 750ti:
750ti is better
Faster effective speed, faster multi rendering, faster NBody calculation and faster complex splatting
The R7 is a potato, the 650ti isnt total crap but its decent-ish, but out of all of them the 750ti is better. Keep in mind these arent exactly the best cards.. But if you save a bit more money you should get at least an R7 260x, its better than the 750ti.