I am a retired AVS tech and can give you a bit of info based on my career. First off going to flight engineer from AVN is the way to go as it is would certainly give you a solid background for FE if that is your goal.
I spent most of my career on fighters, CF-104 and CF-101, these aircraft plus the CF-18 have fighter control radar, computer systems and weapons that are complex and interesting. The aircraft themselves are front line fighters, high performance aircraft. Google can provide lots of info on this subject. Search around this site and you will find discussions by CF-18 techs and pilots.
As your AVN friend said from his experience on transport aircraft AVS looks fairly limited and boring which is somewhat true as the electronic systems are fairly straightforward in their purpose, navigation and communication. Nav and comm is needed on fighters also but the big challenge is the more advanced electronic systems on the aircraft. Read up on the CF-18 and other modern fighters F16, F22 these are the type of systems you will work on. If you love learning, AVS exposes you to a broad number of electronic systems and the computers to integrate them
Both are interesting trades and if you like aircraft as I did they make for a great career.
Good luck!