There is absolutely no reason for a DO on scholarship to take the USMLE. I will give you several reasons.
1. The military accepts both and really doesn't differentiate between them
2. If you get to defer to civilian you can still apply to all DO residencies and most MD residencies, with exception to some of the really competitive ones and some surgery ones.
3. Your training may hinder you. I have done a lot of reading on this. DO education, especially the first 2 years is much more clinically based than MD education. The USMLE has a lot more emphasis on Histology, Biochemistry, and to some degree Neurology and Immunology (at least in the heavy details) than the COMLEX does. This is why DO's tend to score lower on USMLE than MD students do, they are traditionally scoring worse in particular areas of the test. Likewise I would expect if MD's actually took the COMLEX they would score worse on the OMM (obviously) and also probably score slightly lower in behavioral medicine, and physical diagnoses areas (the USMLE uses more "buzz words" than COMLEX does). You will see "moon facies" in USMLE, you have see it in COMLEX as well, but you are more likely to see "round, edematous face"....