It depends of what you like. If you have no previous military experience: Go Air Force. Most civilians say that it's the best of the Armed Forces in terms of caring for you as a person (family, etc). I love the Navy, I love getting underway, going overseas, I can't imagine my life away from the ocean. I'd think I'd die if I had to live anywhere far away (longer than a 20min drive) from the beach. I just got commisioned as an Ensign in the Navy and I'll be attending NYCOM for the next 4 years. I spend 5 years as enlisted in the Navy and I am so happy I got the Navy HPSP.
What do you like? Do you like the water or do you get sea sick? Do you like planes or ships? If you are not sure about spending 6 months in a ship underway in the persian gulf or Europe, then GO Air Force! Best of luck! And if you end up chosing the Navy, you can PM me or e-mail me and I'll be happy to talk to you a bit more about how great my experience in the Navy has been.
I love the Navy, but I'll be the first one to tell you that the Navy is not for everyone.