If you are going to shoot 55gr - 62gr ammo, go with the 1 in 9. If you think you will ever want to shoot some 69gr or heavier, go with the 1 in 7".
I think the 1 in 9 barrels are a lot more common. I shoot 77's out of my 1 in 8 competition rifle. I have shot 69's out of a 1 in 9, 16"...