I'm pretty sure Mossberg 500s are completely interchangable. If you buy the least expensive model, you can change everything else on them.
However, Mossberg does have some new 2011 offerings that are pretty impressive.
Here's their latest tactical 500 with an M4 style stock that holds extra rounds, a ghost rear sight with their orange front post, and a tricked out tri-picatinny rail forearm for lights, lasers, and vertical foregrips.
The 930 semi-auto tactical has a fantastic LPA rear ghost ring sight and a super easy to see fiber optic red front sight. Recoil with slugs and 00 buck in the semi-auto is easily manageable.
The 500 kicks pretty hard with slugs and 00 buck, but that's the nature of the gun. I shot a slug out of my pistol grip only Mossberg 500 once. Once.