I'm a DO, I'm in my 3rd year of AD payback as a flight surgeon, I took both and I wanted to do Radiology when I was an MS4, and the military sent me on this route (don't get me wrong, I love it.) You're going to hear a lot of people tell you to just take the COMLEX around here. It really depends on what you want to specialize in, but even if you only wanted to do Family Medicine, you really don't know what the future may hold. I was one of two DO HPSP students in my school, class of 20~, that took the USMLE, and did well. It's opened up opportunities for me 4 years after the fact now that interviews are starting to come in, at some programs I never even dreamed of being able to set foot in. Just take it and keep your options open. It's the same material so there's no such thing as "studying for two different tests". If you do well on the USMLE, your COMLEX scores really don't matter.
Whether you are in it for 4, or in it for 20 years, you always want to keep as many options open as possible. Compared to my classmates in similar positions, it is nice to have that flexibility in deciding whether or not to stay military or not. I don't understand why anyone would ever limit their potential.