Have you tried any of those bikes yet? If you haven't I would go and try them or, try as many in your price range as possible. It is possible to get disc brakes for less than 500 but they wont be very good disc brakes. For just about 500, or maybe a bit more you should check out a Specialized Hard Rock Disc. I think for the price range it has the best mix of parts. Try it before you buy it though, you want to make sure it fits you nicely.