I agree w/ Sether
COMLEX = heavier on Pharm, micro, peripheral anatomy and of course OMM
USMLE= heavier on Biochem and molecular
I am taking the USMLE first for several reasons, with the major one being that all I'll really need to "add" b/w the two is OMM. If you take the USMLE first you are going to be well versed in pharm and micro anyway. That being said, I think it will take much more time and effort to review Biochem and Molecular, so I'm taking the USMLE first...studying OMM for a few days...then taking the COMLEX. If you try to do the reverse, and study Biochem/molecular b/w the two.....IMO that is more work and can cause you to forget some of the major subject matter (ie Path and Physio).
Hope that makes sense