I am somewhat interested in this as well, depending on debt and such. Army and Air Force promote faster than Navy(6 months), but if you join the Navy after 1L and are commissioned as an Ensign(assuming you're sure you want to do this), you get service credit of two years which will increase your pay more than your Army and AF brethren. Plus, it's only another 6 months later until you're promoted to O-3. Navy JAGs have the opportunity to be deployed on a ship as the legal officer. From what I've heard, you get great perks, as you're automatically a department head. Also, there's less busy work.
I think I'd pick AF if I wanted to be various places around the continental US. Navy means you're in one of the major parts or onboard a ship, and get a cooler uniform IMO. (Hey, you're going to be wearing it most of the day-it might as well look good)