Barrel length doesn't affect accuracy. In fact, a shorter barrel may tend to be a little more accurate for a given profile, because it can have a little less whip. Barrel length affects velocity. Lots of pieces to that conversation.
A 1:8 twist handles 77 grain Matchkings just fine. That's about as big as you're gonna get if you want your ammo to fit into a magazine. Anything bigger, and you gotta run the gun in single shot mode.
The trade-off is always accuracy at a given range vs the weight of the gun. For hunting, you gotta add in the amount of smack you are going to have at a given range. I wouldn't fret too much over a 2" difference. Depending on how they're chambered, the 18 could wind up being as fast as the 20. It's also probably gonna be lighter, depending on the profile of the two.
Best thing you can do for accuracy is to free float the barrel, and then tweak your loads for optimum barrel time. Oh... and use good bullets.