I'd look at either a Remington 870 or a Mossberg 500/590. I think that the main differences are the location of some controls (safety as well as slide release) and the shape of the stock. When I was originally looking at shotguns it was the stock shape that made the slight difference.
Something to keep in mind is that it is easy to upgrade the magazine capacity on the Remington but you're pretty much stuck with what you buy with a Mossberg so get the large capacity mag if you go that route (I did).
A side note on this, the Mossberg M590 holds 1 more round than the M500 with equal length barrels, don't ask me why. The M590 also has a metal safety (vs the M500's plastic one) as well as some other metal vs plastic parts. Oh, you can also get a M590 with ghost ring sights instead of the standard bead sight. Yes, there are times I wish I'd gotten a M590 but I wanted a shotgun right then so...
The 18" barrel is the legal minimum but many shotguns seem to have a 20" barrel as you can get a little more magazine under them.
I'm not sure on the Remington but the stock recoil pad on my M500 was simply a chuck of rubber that had minimal to no compression abilities. A Pachmayr recoil pad fixed that. ;D
I'd suggest handling the basic models as your local gun shop but here's a link to a online place that has some fairly good prices on used Mossbergs.
http://www.jgsales.com/index.php/ri...9_302?osCsid=60dd0afedee0f64aa95148af570d1f1a
The various reduced recoil loads (aka tactical loads) are much easier to handle. I've tested an assortment and they are my primary defensive loads for 12ga. The Remingtion 00 Buck 8 pellet one feels just like cheap birdshot when fired. Based on what I've heard and read I have no reservations using or recommending the reduced recoil loads for defense.
I wouldn't mess with anything other than 00 Buck (well I'd consider 000) as there's a reason that it became the standard 12ga load over decades of law enforcement use. And with full power 3" loads, full power 2.75 loads as well as the reduced recoil 2.75" stuff there's a bit of choices out there as well.
Oh, something to stick in the back of your head. Aquila is/was making some shorter shotguns shells (called mini-shells) that look interesting until you read up on them and find that they only work in a couple of brands of gun due to the feed mechanism differences. And I don't think that Rem/Moss were on the short list.
Steelheart