Big difference. Gau-21 has a much higher rate of fire and a much lower failure rate. It's basically an improvement to the GAU-16.
http://www.navweaps.com/Weapons/WNUS_50cal-M3M_MG.htm
http://www.navweaps.com/Weapons/WNUS_50cal-M3M_MG.htm
Given helos again, HSC
Was HSL
hsc is becoming a different animal, slowly. A lot of the old school HC bubbas are fighting the shift towards a more tactical based mission. A lot of the guys in that community went HC/HSC because it has a rep of being laid back and easy going. They don't like having to focus more on tactics.
As an HS guy, I'm partial to carrier aviation. HS squadrons seem to have a tighter wardroom, mostly due to the small size of it. HSL and HSC is det based and have huge wardrooms. Those two things tend create a different atmosphere.