If you can ignore all the inter-service willy-waving that a question like that is bound to provoke, I think it boils down to a couple of questions:
1. What do you dream of flying? If you're a helicopter kind of guy then look at the various types and roles that each of the services have. But don't overlook joining the AAC to fly Apache. If you want to fly big planes, with one eye on getting your airline licence, it's got to be RAF. If you want to fly jets, either Typhoons or JSF then you probably stand more chance of doing so in the RAF, but do some research.
2. What do you think about going to sea? As an RAF chap, who's done 9 months at sea, I think it's a miserable way of life, and a miserable way to restrict the way you operate your aircraft. But it does have some advantages and it does appeal to some people. If you're one of them, then the FAA is as good a place as any, and their uniforms are much sharper than the RAF's. All RN and USN aviators will wax lyrical about the joys of landing on a boat. If you end up on Harriers, or JSF, as an RAF chap, you'll land on a boat too, and won't think much of it.
And now for some willy-waving:
Tourist,
It seems that you've taken some kind of selective memory pill. Perhaps you'll recall from GW1 that RAF Tornados were specially requested by the US to go in low and close the major Iraqi airfields on the first few nights of the war (you can argue about the necessity to at great length, but it was a capability the US didn't have). Having operated with a US unit during OIF I can tell you that the UK tanker crews are known as the best and most flexible AAR force in the world. Our AWACS guys are a regarded by the US as far far better than their own. Our SF Herc and Chinook force is envied. Our Storm Shadow missiles were specially requested on several occasions by the CAOC. Our EPW bombs were acknowledged to be better than a plain old JDAM. And our own land units would much rather have an RAF CAS aircraft overhead than a USAF one, given the choice. The Junglies did a great job at Al Fawr, but to suggest no one else brings anything to the party smacks either of ignorance or absurd arrogance.
Single Seat, Single Engine, The Only Way To Fly