USMLE is harder than COMLEX, and don't let anyone tell you otherwise. The numbers support this assertion. Also, most osteopathic curricula do not prepare you well for the USMLE. I personally didn't really give a s**t about doing well in classes while a medical student. I used to skip class, and played a TON of mariokart. I spent my time wisely, studying for the USMLE. I read a lot of Robbin's, and a whole lot of board review books (Kaplan q-bank, USMLEworld q-bank, Step-Up, first-aid, and the majority of the BRS books, as well as several buzzwords audio CD's.)
If you can pass the more difficult test, the easier one should be no problem, right? i.e. if you're going to run both a 5K and 10K race, are you going to train for the 5K and then wonder why you struggled with the 10k? Of course not, you'll train for the 10 and smoke the 5.
I would recommend using some of your vacation time to study for USMLE. I did, and it served me well.
You will be viewed as more of an equal to MD applicants if you have taken the USMLE and done well on it.