All the Boss machines record in compressed data, so your sound quality takes a defiinite hit. The BR-8 has more bells and whistles, but the bells and whistles are utter crap compared to what you can get for free in software plug-ins, so...
I owned a couple of Boss products before the mr8 and while you can technically "do more" with the BR8 from a standalone perspective, the bottom line for me is the sound quality, especially since I do my mixing on the computer. So I suppose it depends on where you're planning to do your mixing. If one of the deciding factors for you is Boss's "amp modeling", let me warn you: it sucks eggs.
Cheers,
Chris