I own and use both twists and love them both. My range/training ammo is 55/62 grains, and I can't perceive any difference in accuracy with either twist. With decent ammo both twists are far more accurate than I am.
If I had to choose one I'd probably go with 1/7, simply because I know that my defensive ammo of choice (75gr TAP) will work with it. 75gr will probably work in a 1/9, but it will most certainly work in a 1/7. Both my Rock River and Colt 1/9 barrels seem to shoot 75gr ammo fine, but I have a 1/9 barrel of unknown origin that doesn't group worth a damn with it-though it's great with 55 and 62gr of any flavor.
That being said, there are other more important choices to consider in an AR than barrel twist-at least to me anyway. I'd rather have a well built AR in 1/9 than a shoddily built AR in 1/7.