The COMLEX is just the USMLE plus a little extra OMM, so I think it's worthwhile to prepare as if you're going to take both...which means studying harder than you ever have in your life. ;-) Then, as the test date comes close, take one of the official USMLE practice tests to gauge how well-prepared you are and if you can do well on the USMLE. If not, just cancel your appointment for the USMLE.
Personally, I took both and actually did BETTER on the USMLE than on the COMLEX (mostly because I dislike OMM and didn't study it very hard). Even though on the day of the USMLE I was extremely nervous and tempted to back out because I felt like I wasn't ready and never would be - in the end, I am definitely glad that I took both. 🙂