HI guys
I've been an aircraft avionics mechanic for the past 20 years, the first 9 in the RAF. Since then I've been a contractor on a variety of rotary and fixed wing platforms, both civilian and military. Hopefully I can give a little advice, in a few short sentences, and help you decide. (Point to note, I've no experience of the AMM thing the RAF do now, they were FMLs in my day!)
Generally the trades are known as light or heavy, light being avionics, heavy being mechanical. There is a reason for this. :-) Avionics guys will deal with electronics and electrics, communication systems, navigation systems etc etc. If logic gates and wave guide theory are your thing, this is the trade for you. Some may also say uckers also features highly on the curriculum, I couldn't possibly comment ;-)
Mechanical guys will deal with engines, airframe, hydraulics etc. Dirty, greasy, oily creatures, they'll be the ones using AVTUR as aftershave, and some have even evolved to stand upright and communicate using a primitive form of speech. Often over worked, and under appreciated, they move in herds of 6 or more.
After you've finished with your RAF time, you may decide to continue into civvy street, and go for the big bucks as a contractor. You'll be required to purchase your own tools. It's easy to spot avionics contractors as we carry around our tools in a small handbag, whilst mechanics generally lug around a huge snap-on tool chest, full to the brim with every tool imaginable, and in debt for the rest of their lives to pay for this. Also worth noting, Avionics often earn a couple of quid an hour more. (This has become less common in recent times however).
Either way, you'll enjoy your time in the RAF as an aircraft techie. All other trades will look on you as gods. At the end of the day, the RAF is about aircraft, not shiny arses ;-) And your training will stand you in good stead for many years to come.
I'm going back to uni to study something completely different, but will still subsidise my studies with contract work.
Good luck in your future careers. Per Ardua Ad Astra.