I started out with a 28" barrel on my Mossberg 500, then switched to a 20". It still shoots to point of aim and kills turkeys reliably. It's a lot nicer to carry, especially when its riding on your shoulder on a sling and not catching on every branch overhead.
I prefer a 20" barreled shotgun in the woods for the same reasons I prefer a 20" barreled carbine in the woods instead of a 28" barreled rifle.