I agree with Mtn Creek, look for a larger main tube and a mil-dot type reticle (some of the Horus style ones are great, but an added cost). With the two, you'll get increased dialing range and some additional holdover if you run out of room. Like he said, do your research, because you might decide you want a base with some elevation built in to buy you a little extra room.
Other things to consider:
Parallax adjustment is pretty much a must. If you're almost always going to be shooting from a seated position, the style won't matter as much, but if you're going to shoot prone, make sure the controls are easy to reach.
Zero-stops on the turrets are nice, but not really a must, unless maybe in competition. Same with clicks that match the subtensions on your reticle. If it's a mil-dot, try to get turrets with mil clicks. You'd be surprised how hard that actually is to find.
Decide if you want first or second focal plane for your reticle position. FFP gives you the ability to range and do holds at any magnification, but some people don't like them because they "appear" to grow and shrink with magnification. This can be exacerbated by lower quality gear. SFP, on the other hand, can only be used to range at a specific magnification. Some benchrest guys prefer SFP, and for legitimate reasons, but most other long range shooters prefer FFP.
Needless to say, get the best quality glass you can afford.
As Mtn Creek said, a 12x or 15x fixed power would be worth considering. With good glass it should let you see well enough, but it should be, all things equal, less expensive, and you won't have to worry about the focal plane. If you use it for hog hunting, though, it will likely be too much magnification.
I wouldn't steer you away from any of the brands mentioned, though with some, like Nikon, it may be hard to find all the features you want. I hit the top end of your price range pretty quick when looking for my last one. You may decide you can live without one or two features if it stretches your budget too much. I think you have the right mindset of spending on one good one rather than several cheap ones.