Ive said it before, but here are my thoughts:
1) I took both tests, because I am interested in a competitive specialty.
2) I took them 17 days apart. I feel that I needed this time to decompress from COMLEX (which ultimately was the important one) forget my OMM and micro, and rework pharm, biochem and stats. I think it made a BIG difference, if only because I know a lot of people who took them back-to-back and never felt relaxed.
3) Only take USMLE if you think you can do very well (i.e. beat the mean). I think it is silly to pay the $400+ for the test if you aren't going to do well on it. I would rather take a good COMLEX score and $400 then a decent COMLEX, average USMLE and the problem of whether to submit the score from the test I spent $400 extra to take. It is a quite different test from COMLEX, and if you arent ready for it, it can be trouble.
4) The USMLE was plenty clinical-oriented. However, it focuses more on things that are actually written down in books somewhere. Not to slam the COMLEX, but it focuses more on things people tell you...perhaps because the research isnt always there. Sure, you get a question on Michaelis-Menten in USMLE, but it isnt that bad, and isnt it important to understand Vmax?
5) In the end, I think these tests are quite different applications of much the same material. It can be difficult to do both, and I would make sure that you are ready before you do. Just dont blow one off or assume that because you are ready for one that you are ready for the other.
Just my 2 cents.