I think 22" may be ideal. No doubt more convenient in thick cover, and honestly, if you're in thick cover you're likely taking a shot inside of 30 yards; so it's not like shot pattern is as critical, plenty of loads and chokes out there for a good pattern inside 30 yds. I hunted turkey in earnest for the first time this year with my old 28" Maverick 88. I really didn't find myself wishing I had a shorter barrel though, and enjoyed the added sight radius (using 3-dot adjustable).

Also, it irritates me how the market is - it seems like the "turkey" guns command a premium compared to the equivalent "field" gun setup with a longer barrel. I know there is a lot of voodoo involved in the making of a good shotgun barrel, but it really seems like you're paying mostly for the trademarked camo finish. As a new turkey hunter, if I had to recommend gear to someone else getting into the sport - I'd say buy the cheaper, longer-barreled 870/500 and spend the extra change on some 3-dot adjustable sights, a good choke tube, ammo, and a handful of Woodhaven mouth calls.