I have built them from 20" to 26", all were AR's. What I found was it makes more of a difference what you are going to shoot than anything. The 1:12 twist barrels that I used I think a 22" barrel would do just about everything you could do with a longer barrel. But I have built the .204 and the .20 Tactical with a 1:10 and 1:9 twist and have found that an extra couple of inches makes a big difference on the heavier bullets.
No difference than any other caliber, you have to decide what you are going to shoot, and what is important to you.
There is a very fine line between "Hobby" and "Mental illness"