took both last summer and did well on both. basicly they cover the same material with a little different emphisis. comlex hits more pharm, micro, path while usmle will hit more biochem, pathophys. but there is significant overlap.
comlex qs are usually shorter and more to the point while usmle make take you a few secs to figure out what they want and what pathway they want you to think about.
example:
Comlex question: Guy comes in with cough after seing a Dr. for hypertension. What medication is he probably on: Answer: pick out the one with a pril on the end(Ace inhibitor)
usmle: guy comes in with cough after starting an ace what electrolyte is most likely increased: answer K+......thought process=ace= dec ATII= dec. aldosterone= inc K+
Having only taken the USMLE, i can't say for sure. But my suspicion is that they *both* test a combination of memorization and reasoning. Why would they be so drastically different?