I'm going to get the H&R Ultra Slug Hunter 12 ga. for next season. I would definitely consider this as an option. I have read and heard many good things about this gun...and it only $250 brand new. Can't beat it. Some drawbacks-very heavy because of the bull barrel, and its only a single shot so if your a guy who needs more than one shot it might not be the one for you.
I currently have the 870 and I'm not really happy with it. I think it's because I have a red dot scope on it instead of a shotgun/muzzleloader scope. I haven't been able to get very good groups with it, and you have to re-sight it in everytime you change your barrel. Like BUCKMARK, I have the saddle mount but I havn't had the same success as he has with it. I would recommend getting a cantilever barrel with it if you are planning on switching out the barrels.
I have a friend that just bought the Mossberg 535 Turkey/Deer Combo. It comes drilled and tapped so you can put a scope on it and IMO it would be better than the 870 with a saddle mount. He is pretty happy with his purchase. I think he got it for just over $300 for just the gun.