Hi Chance.
Form a strictly engineering standpoint, a 6x8 is stronger than a 7x7, at least in bending and deflection. Although intuitively I agree with Mark that either is strong enough, one should do tha math that takes into account the rafter spacing and span. Personally, I prefer the look of a rafter with a rectangualr cross section over a square one. Good luck with your project. CB.
