i prefer mountain biking much much more. you can go anywhere and do anything! like skiing you are expected to develop skills in all the facets weather it is trail riding, downhill, tricks, jumping, you have to learn a much wider set of skills. bmx is sweet they do a lot harder tricks. but now freeride is picking up alot of those and doing them over 50 foot gaps so it is a little more dangerous.
oh by the way i have a 04 santa cruz bullit that is completely decked out i gaurantee better than your roomates for 2,800 if you want to buy it. the price is low because it is a little over a year old but i have kept it in perfect condition. if anyone wants it e-mail me or message me and i will get you pictures and info