Listen to your heart, but don't pospone your decision. If you don't decide now, you might hurt both. How would it feel to your gf knowing that you're into someone else, and how would it feel to the other girl that you're only fooling her since you're already in relationship?
Choose the one with which chemistry works more, and consider for which one would you do more, accept their mistakes and even in hard moments, try to make things better and not only care about self...and which one would do it for you?
i think you'll see what happens next ^^ i mean if the "other" girls is really interested in you she won't let it drop, and if you love your girlfriend you won't let her drop ... so maybe you need time...
i'd add that you always know what you lose but not what you'll get
I'm a military brat and I've plenty of my buddies (who joined after highschool) whose relationship get's torn apart because of deployment. It will happen to me too, as I plan to enlist after college. I think the heart only has room for one significant other, you seem to like the second girl and you've already been with your current girlfriend twice before and those times plus this one has resulted in some kind of problem in the relationship. Don't you think it might be time to just let it go with her? That Johnny Depp quote in of the comments posted here is great. Just remember you'll get deployed and a lot of girls can't stand that type of distance for so long. Good luck, man.