The OD808 sounds better as a true clean boost, IMO, and the OCD has more gain.
I've owned both, sold the OCD, still have the OD808, which has since been PWNT off of my pedal board by the Suhr Koko Boost, which is hands-down, in all my years of pedal whoredom, the best boost I have ever tried. Whether you use it for a true clean boost, or employ it as more of an overdrive, it just can't be beat, IMO. The mid-boost feature for me is really what makes it!
I see Mark, aka samhill, is a mod here, ask him about it and/or check out his demo vid of it sometime!
EDIT: Nevermind, I'll just go ahead and post it here myself:
If you really want a Maxon, though, I'd recommend the OD9. Essentially the same basic concept as the OD808, it's a killer super-screamer that's built more ruggedly. I've had the switch on my OD808 go out twice on me, and if you check out Analogman.com, Mike will also recommend the OD9 for the same reason.
They're all great pedals, though, and owner subjectivity will likely be the main difference in the end.