My big issue when I had to decide between AF and Navy was location. With few exception locations where I would be training and serving (i.e., locations of major training hospitals and then branch hospitals) were near the ocean, both in the US and around the world. It is the Navy, after all. Living in wide variety of climates, being on the water is much better than not for me. Being ship-bound/ assigned to a ship doesn't mean necessarily that you will be living on a ship for a year's time. Very possible that you will deploy for a decent amount of time, but you will be stationed at a home port where you live a 'normal' life when not on deployment.
Also, unless you have a very finite life schedule, the Navy is a great choice specifically because of the GMO possiblility. I have a ton of experience in the ER before school (EMT/ERTech/MedAssist, 12,000+ hrs) and am very interested in ER residency. I do know, however, that by the end of 4th yr. and even after internship, I may feel very different. I may also feel even more sure about ER. Either way, you really need to have a solid grasp on what you want for residency and where you want to apply for externships, etc by the end (at the latest) of third year. This gives you only one year of clinical exposure to decide what you want to do for the rest of your professional life.
By going GMO, whether that's flight surg, Dive med, Fleet Marine Force or good ol' plain GMO, you have 3rd yr, 4th yr, internship, then a year or two of practicing medicine (albeit at a primary level) to really decide what you want for specialty. r90t had a great point when discussing the GMO thing. Places he interviewed at for radiation oncology while on his GMO tour (he's going to USC on the Navy's tab) were very interested about the fact that he had as much primary care experience as he had. From what he said, it was warmly received and a definate plus for him as an applicant.
I am in no hurry to jump into a field that I hadn't fully considered and subsequently achieve burn-out early in my career.
That's how I chose, hope it helps.