Definitely keep what you have. first thought is, you have no clue where your even going to be stationed yet. If not stateside, the car would sit for awhile until you get your driving licence and such for that country.
And never ever buy from the shysters around a base, I've seen too many younger guys get sucked in way over their heads with way to big of payments and too high of insurance.
best bet as someone said, wait till your settled and if you have your heart set then go on leave and trade yours in locally for what you want. your far less likely to get screwed by a hometown dealer.
But being an e3 at the bottom, keep what you have and bank the money you save toward a large downpayment later on. don't jump in with bigger dreams than your pockets allow, thats a good way to get yourself put out on the street without an honorable discharge.