CA come stock with metal bushings and reinforced mechboxes, TM don't. Armalites are dead easy to find parts for and you can swap mags with team mates in the field in a pinch since more people use armalites. I am selling my CA AUG and bought an ICS L85A2 so I can use the same mags as my M4. M4 mag pouches are also easier to find, a G3 would require 7.62m/M14 mag pouches which only some companies make in a smaller selection of colours and types.
Personally I would go with a G3 because I just love 7.62mm guns and already have the pouches and such it would need because I use an M14. But if you are new and looking for sometime that is easy to get parts on and that more people know how to work on to help you upgrade/fix things then getting the CA M15A4 would be your best bet.