Ac-130 and i'll tell you why, the first reason is because it has a three weapon system. It does have reloading but if you use it stratigical you can get more kills with it. Me personally, im terrible at using Chopper Gunner, for some reason I cant get the fucking thing to shoot straight. I get like 2 kills. I also like Ac-130 so some noob won't go shoot it down withen like 20 seconds of use. It's harder to get, yes, but its worth it.
IMO it rly depends on the map and game mode. For example, Wasteland if good for both air support becuz chopper gunner its easier and faster to kill ppl. But when using AC-130, its BETTER when playing on domination cuz spawn trapping with AC-130 does a lot of sh*t. Personally i like the AC130, soley cuz its fun