The FM series is really great I'm sure you'll love it. I suppose the main thing is how bothered you'd be about the database being outdated on fm14. If you want all the most recent transfers etc splash out on fm15, but you'll easily get plenty of life out of fm14 if you'd prefer to be a bit frugal - I imagine as a new player if you really enjoy it, it will take you through comfortably to fm16 in November.
Game wise there will naturally be an improvement from one year to the next, but as a new player you won't notice the little changes between each iteration really and FM14 is a fantastic game so you won't be missing out on much if you chose that option.
Personally I'd go 14 if you want to give it a trial period so to speak