Look at the data. The primary difference between a 3" "magnum" load and a 3 1/2" "magnum" load is the total mumber of pellets in the shot column. Muzzle velociy is about the same. And it is the choke that shapes the pattern that you want depending upon what your preferences are. Either choice is plenty for large fowl such as geese or turkey ... I'd say out to the 40 +/- yard range no doubt.

Add a rifled slug barrel to a 3" "shot gun" and you're looking at being able to take deer sized game cleanly out to 125-150 yards.

The recoil of a 3 1/2" shell is severe to say the least, but some folks don't mind it. I for one do!