Hello Chaps,
Personally I think it depends on the kind of flying you want to do, I have friends that have flown in the Forces and loved every min but get very frustrated when the leave to find that they have to be F/O for 7 years to get a long haul command. (That is an average of my friends anyway)
You have two fantastic options, personally I prefer the Airline route to Mil but that is becuase I am from a family of commercial pilots and not for the money as some might say! If you cant decide why not do them both and get free hrs, good experience and good fun flying in the Navy first. I would be carefull though, when I did the IR there was a guy who flew the sea king and as it was rotor time and Navy he had to complete the full ATPL upon leaving unlike some guys from RAF that just have to do a conversion. I dont know if that stands with fixed wing flying.
Best of luck either way it will be great fun!