Does the "other" choice include "all the above"?
Really, get whatever ya want, like, feels good to you, and what you'll actually practice with.
That said, I think one of the earlier replies nailed it with an AR-10 of some flavor in .308. Here's my reasoning:
You already have a shotgun, and with slugs, it should be able to stretch out to 100 yards or thereabouts alright. With buckshot, it should handle things up close & fast. So if you're gonna buy a rifle, I wouldn't bother with something that barely stretched your effective range. Get something in a real rifle round, that can accurately deliver smackdown way out there. A semi auto will greatly increase your chances of taking another piggy or two before they all run off, or deliver a quick follow up. And I think its advantages would be obvious in any kind of defensive role, especially when ya consider any aggressors would likely be armed with semi autos of their own.
Why the AR-10?
Honestly I'm not a big AR fan, even though I've got a couple. But the price range you mentioned rules out a lot of other patterns like the M1A, while AR prices are at rock bottom right now. The PTR-91 is another solid choice, but I think the AR would give you more flexibility on mounting optics or night vision later, and they usually cost a little more. The FNAR is worth a look, and would probably be more familiar to you if ya come from a hunting background since it's based on the Browning BAR hunting rifles, but you might have to shop around to stay within the budget, especially if ya want a whole pile of mags. For a long time they came with a MOA guarantee from the factory, so it should be a shooter. The FAL is known for rugged reliability, but not known for accuracy.