The tests are fairly similar in format. I didn't notice one to be more 'clinical' than the other, but then again I didn't pay much attention to that. I thought COMLEX tested a lot of low-yield material...first aid was not very helpful in my opinion. The USMLE tested a lot of stuff out of First Aid as well as things I remember being stressed as important in class. I felt the COMLEX had a ton of cardiac pharmacology on it compared to other subjects whereas the USMLE seemed more balanced. I also felt the USMLE was written better...I remember thinking there were a number of bad questions on COMLEX and I didn't feel that way after the USMLE.
The USMLE lasts one day because it is only 350 questions. COMLEX was 800 questions, so it took 2 days to get through. COMLEX is paper and pencil and USMLE is on the ocmputer, so you can schedule it whenever you want. I took COMLEX on June 5th and 6th and the USMLE on June 8th. Pretty much everyone I know has already received their USMLE scores (if they took the test in June) but COMLEX is making us wait until at least the first week of August....very frustrating.
I thought the COMLEX was more difficult because of the things I have already written about. This isn't to say that the USMLE was easy by any means...in fact I thought I did better on it than I actually did. In general, I thought USMLE was a better test of our ability to function in the hospital and I thought COMLEX was a test of how much little stuff we remember. That's just my opinion though.