Ditto on what Mckinley said. Go with what feels right. I normally just twist in the mod choke and leave it in, with the exception of turkeys where I want a full choke, mod seems to work the best for most of what I do.
Any particular reason for the supermag? I have the Mossberg and Rem in 12 gauge 3" and never felt undergunned even for honkers. Big attraction for me is the 3" factory ammo is less pricey than 3 1/2" and I can set my reloader for 3". If a supermag is what you like though, by all means go for it.
Safe shooting.