I'm thinking of reenlisting into the National Guard, but change my MOS to aviation (15N or 15G). How good are these MOS's outside of service?

Either MOS can get you an Airframe and Powerplant certification to work on civilian aircraft, which is becoming more of a lucrative field. As an Avionics person myself I can tell you that Avionics is more rewarding, and from what I have heard the training for 15G has a higher fail rate.

I was in NGUARD has a aviation structural Mechanic and hardy ever did anything to with the structure of the helicopters I worked on . But transfer from the Naval Reserves and that was in the early 1990’s it may be different now but do what think is best but in my opinion the Army and NGUARD aviation isn’t that good.

As a prior 15G, if I had it to do over again I would have chose something in the cyber security field. This is a field that with experience you can walk out making 130k+ using your mind and not your body. I won’t say that I don’t miss working on aircrafts, but the cyber security field has been very lucrative for me. Additionally, the civilian sector requires you to get A&P certifications for a role you have done most of your military career. The national guard / army should provide this as standard training.