You might want 3.5" if shooting geese with steel.
I got the 23" turkey model Supermag 870 and it's my backup rig.
Imp Cyl choketube it ran right at 50% on doves last yr at a F&W drawing (steel #6).
Year before, 2nd day (also drawn) it ran 9 in a row before I got stoopid (still shot 50%).
Guy with the Redlabel and shorthair to my right (most birds coming from him) packed up and left.
He was having a rough day, and my cheap "wrong" gun was doing pretty good
The guns are the same really, receiver length the same. There is an enlarged ejection port, some trigger group and action bar differences for the slightly longer stroke. If you aren't a regular 870 purist you won't know the diff in feel.
I've had 3 Supermags and over a dozen regular mags. No probs.
Do prefer bismuth #2 in 3" and an autoloader for geese. Makes triples easier
To keep cost down, steel in 3.5" might work...........but back in the day i hated the results. Brings honkers down most times, and when they hit the deck they often aint out of the game.
That was with BBB steel, not any of the hex or cutter types of pellets.
Got some Blindside BB to try on goose in 3.5". Turkey loads I just run 3", HV #4.... inside 40 yards it slams em.
3.5" steel loads I don't think to have much recoil.