I'm fairly new to MTBing too, but have done bike stuff my whole life. I can tell you that, in general with bikes, if you do a lot of reading up on parts, and take your time and wait for deals to come along, it's cheaper to build a bike. However, that does take time. I think if you're looking to be on a bike pretty soon, you're best off buying a good condition used bike. Probably CL or something is your best bet.