Commericial Bike kit vs. DIY Bike kit?

I need a decent commuter bike that can take some hits and not look too desirable to the bike thieves.

I'm debating whether I should just buy a cheap complete bike kit (<$200), or invest a little more effort and buy a bunch of cheap parts myself off of nashbar to assemble. I'm not sure which would get me the best value for my money. :confused:

I thought I'd run it by you guys to see if you know of any pitfalls in doing it either way. The biggest problem I see is the fit. The bike shops are unaffordable, so that's why I've been looking online. Thanks in advance!