Well, I took the USMLE and the COMLEX. They both are different tests and though you may study the core subjects the same, there are subtle diffences that lend themselves to using different books.
First of all I did well on both exams, but far better on the COMLEX. THe COMLEX has farrrr less genetics, biochemistry, ethics, and stats than the USMLE. And because of this, FIRST AID for the USMLE is an absolute necessity if you wish to score the mean. ANd aiming for the mean is something that you will probably will want to do.
The COMLEX has far more pharm, musculoskeletal and microbiology...as well as silly nit-picky questions that are good only for medical jeopardy.
I personally think that STEP-UP for the USMLE lends itself to better preparation for the COMLEX due to its integrated approach and excellent chapters on musculoskeletal, respiratory, and cardiovascular systems.
Unfortunately, it is exceptionally weak in pharm, because of that, i would use the following:
1. Pharm Cards
2. High Yield pharm
3. First Aid
Other books that supplement both exams are BRS Path, all in the High Yield Series, and Clinical Vignettes.
In particular, I found medschool.com to be a great web site as well as the webpath website from the university of utah.
Do MANY questions and if you are taking both exams...I would take them no more than a week or two apart.
good luck