I agree, either method has the same THEORETICAL potential for accuracy. It's a question of finding what works for you. I'll add this observation, and invite others who get much better results than I do (I'm a pretty mediocre shot, myself) to agree or disagree as necessary.
I find that OTT slingshots seem to have more variability from one frame to another depending on tip shape, tie placement, fork angle, ect. All the things that make one fork different from another seem to have more of an effect on where the shot actually ends up going on a OTT fork, where as a TTF frame seems to be much less sensitive to this. I don't know if this is because of a difference in the way the pouch travels after release, or if it's because there seems to be less variability in the design of TTF slingshots that I have shot (meaning that all the TTF frames I've made or used had the forks parallel to each other and the sides rounded, where OTT can be angled or straight, rounded or squarish)?
But my observation is that I can switch from one OTT fork to another and the center of my groups will shift, even if the groups remain about the same size. where I don't see this if changing from one ttf fork or another. Neither seems to do anything to help me shrink the size of the groups themselves though. That however is a problem with technique, not equipment. Or to use Philly's metaphor, the problem there is with the Indian, which seems to apply equally well to my archery groups. Oh well.