bodemedia tracks sleep too. Bodybugg does not.
From what I've read (don't have either one) the Body Media Fit system analyzes sleep patterns/efficiency while the Body Bugg system doesn't.
Idon't know that it's fair to say the BB doesn't track sleep though, I think if you wear it to bed it does, but from what I understand the feedback it provides just isn't as in depth. So I guess it's a matter of what you're looking for.
Personally I wouldn't want to wear either one to bed, but then I don't really need a monitor to tell me whether I've gotten a good nights sleep or not - I know.
I only checked a couple of online retailers, but the Body Bugg system is cheaper with this deal than any price I saw for the Body Media Fit system.