It depends on the kind of shoot it is, some guys set up shoots like it's a house clearing exercise shots are 30ft so it could be long for that type of shoot. Some guys setup a military theme with very few clearing type stages so a 20" may work better in those type of shoots.
I like outlaw 3 gun events more than the santioned events. I don't practice for 3 gun, 3 gun is my practice so it's just a way to get out and shoot and have a little fun for me. If thats what you are looking for run what you have, if you want to be competitive then you may want to stick with a shorter barrel. Check out the Brian Enos forum.
5.56, 6.8, 264LBC, SIX5.
The Nextgen of 308 rifles