I am a fourth year and have taken both exams. Generally speaking, they test your ability to study, cram, and vomit information equally but do so differently. The COMLEX is two days of torture. Four hundred questions each day divided into two blocks of 200 questions in a day (800 questions total) The exam was trial by volume, encompassing "buzz words" and common topics to minutia certainly not taught in this continent. The osteopathic questions were easy if you studied for a day or two. The USMLE was a computerized test lasting one day. It required more problem solving, and the difficult questions were generally something learned in your medical career. There are 7 blocks of 50 questions lasting one hour and break time of one hour to be divided at your discretion. The content differs somewhat between the two. I felt basic sciences were tested more rigorously on USMLE than COMLEX and USMLE was easier to take due to a single day of testing. However, I performed equally well on both exams with nearly identical scores. I think this speaks to equivalent teaching, testing and some modified studying.
Good luck,