The faster 1:7 twist will better stabilize the longer/heavier bullets above 69 grains. You can shoot 50 and 55 grain bullets just fine but your best accuracy will be with the heavier projectiles.
1:9 will generally provide better accuracy with 55 grain ammo than a 1:7 rifled barrel. As long as you stay between 55 and 69g your best accuracy will be determined more by bullet type then weight. In other words, match hollow points will give better results than FMJ.
1:8 obviously falls in the middle and although not as popular, is possibly the best all-around choice.
There can be some individuality between seemingly identical barrels so I encourage people to try different ammo in their rifles to see what works best for them.
An amateur trains until he gets it right, a professional trains until he can't get it wrong.