Angled rails came about when those new fangled internally adjusted target scopes were used on match rifles used in 1000 yard matches saying good bye to the externally adjusted ones. A 20X scope may have a 40 MOA adjustment range; that's about 20 MOA from optical zero up, down, left and right.
With a a 30 caliber magnum needing 30 MOA above boresight to hit center at 1000 yards, that scope won't cut the mustard nor give enough elevation to do the job. So an angled rail was invented to take Weaver scope rings. (Jack Davis, gunsmith and NRA Referee at the Nationals made some of the originals). The first ones were 20 MOA but subsequent ones are more or less. They were originally called a Weaver rail. But nowadays it's a Picatinny rail.
One other thing such rails do. They put the scope's optical/mechanical axis better centered in its range so any alignment error between receiver axis and scope axis can be corrected for. With some scopes having less windage adjustment range at their elevation limits, a rail solves that problem.
To pick the one you want, find out what the range minimum and maximum you'll shoot at and the elevation above bore sight needed to do it with yor ammo. Then pick a rail whose angle splits the difference; erroring on the high side if needed. If your scope has enough elevation adjustment to boresight on a 20 MOA rail, that' good. If not, you may need a 30 MOA rail.
For use up to 500 yards, a 10 MOA rail will be best for your scope's optics and mechanics. The optical quality loss at the adjustment limits are insignificant; it's the uniformity of click values in the middle of the adjustment range that are better.
