Range wise, the Super 90. Fun wise, easily the M500. Have owned both and used both, each has its own pros and cons. Put it this way, the only shotgun I have left is the M500, out of 3 shotties.
Super 90
Pros: Light, spring (just keeps going!), good range, manueverable
Cons: Ammo capacity - need shells that are becoming hard to find at a decent price, plastic construction
M500
Pros: Shot spread, loudness, metal construction, ammo capacity, will keep going if you treat it right
Cons: Leaks if not kept gassed, fragile stock charging arms, needs constant attention