For something as small as the 308 I would stick with the 20" tube. You only need something over 24" when dealing with magnums. A 308s "perfect" barrel size is 22" but I wouldn't let a 20" stop me from buying the gun if that is what you wanted. The only case to be made for a 24" tube on a 308 is if you were loading it hot or shooting "light mag" type factory ammo and needed the extra tube length to burn the powder. But if that is what you are trying to do then why not step up too a 30/06 or even a 30cal mag if you are out to juicea 308?

Plus the shortaction and overall compactness of a 308 lends itself too smaller/shorter rifles which are much easier to carry and manuever. I can tell you as I hunt primarily with long barreled magnums, when in the woods in a treestand (or even while clanging around in my shooting houses) I very much wish my big guns had 20"-22" barrels. Slipping through a swamp or brushy thicket is also a PITA with a long barreled gun as well. With a 20" anything the barrel wont even stick up over your shoulder while you have it slung on your back so you don't have to be annoyed by it snagging every vine or branch within a foot of your head!

Performance wise the 308 is equally happy with a 20"er as anything else!
RA