Hey! It's good to be thinking about your options for after sixth form, because you need to choose your A Levels based on that I guess.
It's really what you think would be best for you. The Uni route is an experience in itself, whereas you'll probably miss out on that if you go down the service route. Then again, going down the service route means you'll have no student debt (I think?) and you'll get paid, and have more years experience under your belt by the time you're 21.
My advice is if you want to keep your option open is to at least do Biology A Level (If you enjoy it!) so if you decide to change your mind, then both options are still available to you.
Good luck!