With the FAA, you will have to be very very good to get a fixed wing slot. With the RAF, fixed wing is pretty certain.
So what's wrong with rotary? It's good fun (I gather, and I have done a little bit myself), but one day you will probably want to do something else - and rotary hours are only good for rotary licences under JARs. And I wouldn't chose civilian rotary flying over civilian fixed wing flying.
Plus, you will spend lots of time at sea in the Navy - I gather it loses its' novelty quickly> And in the Navy, you are a second class citizen as a pilot as the main business of the Navy isn't flying. (These last two opinions are those of my best mate who spent 8 years flying FAA and reckoned I had it far better in the RAF).