I got the 23" rifled barrel in a combo right off the bat.
http://www.remington.com/firearms/shotguns/870exp_slug
Last Summer, Gander Mountain had an exclusive combo combining this and a 28" bird barrel, and a decent 2-4xscope.

With Rem Core-Lokt @ 1900fps, it will keep them in one hole at 50 yards off the benchif I do my part. I suspect thehottest loads for the sabots, like the Core-Lokt can make use of a slightly longer barrel for added velocity, but I just prefer the heavy contour and extra length for stability so it is more shootable for me.

Honestly, the 20" balances a bit better in the hand, but the heavier front end on the 23"helps out at longer range.