So for starters, I ride a cannondale supersix on the road, and have been riding for a about 3 years. I want to get into mountain biking, but have only been out once for about 2 hours on a demo bike. I made the mistake when starting on the road to start with a cheap bike and ended up paying much more in the long run to get what I wanted. I don't want to make the same mistake again.
Because of this I'm looking at the 08 rush team, and the 09 rize. I can get a deal on both so they will be close enough in price and want to know which one will be better for me. I know the rush has the lower bottom bracket, but am not exactly sure what that's going to do for the ride.
I am a complete beginner and do not have the best technical skills but am a pretty good climber. A bike that is better descending is more important to me (I think) for know. It will be used for x-c riding, singletrack, maybe some of the all-mountain but probably not too much of that. I may race it once or twice, but want to focus on road, so probably not too often.
Last thing, I know with road bikes it's very hard to do a fit, but I talked to the LBS and they said I should be on a large, but the medium seems like it would be a much better fit if I compare geometries to my road bike. I'm about 5' 10" - 5' 11" with a 32 inch inseam and ride a 54 on the road, which would technically be a size too small, but I like it so I figured I would do the same on the mountain side.

My other option is the specialized stumpjumper, which I don't know nearly as much about, and will say that I am partial to the cannondale, just because it's a cannondale and I know them. I also can't get quite as good a deal on specialized.