Heavier bullets will prefer the 1:7 twist

1:7 means 1 revolution for every 7 inches of travel. The lower the ratio the better it will stabilize a heavier bullet weight

The 1:8 will better run a 60 grain vmax style of bullet while still properly stabilizing a 75/77 grain round. However, if this is for an ar, the max weight you can load to magazine length is 77tmk and smk, so you’re better off with a 1:8.

Simply put, 1:7 stabilizes larger bullets better and !:8 stabilizes smaller bullets better. The numbers are the a twist ratio. !:7 means the projectile will spin 1 full ration in 7 inches.
It depends on what you like to hunt with. A lot of commercial barrels are 1:9, because most people will buy an ar and put the cheap ammo in it which is usually 55gr. I prefer a 1:7 because I shoot heavier stuff. The 1:8 is a good all around twist rate that will stabilize most anything an AR can put out.

Nothing. Unless you want to shoot a coyote with a 90gr bullet instead of a 75gr.