Chopper Gunner is better on *most* maps, though the AC-130 OWNS when you have multiple obstructions in your way of a clean shot that the chopper gunner can't get to. For example...Underpass, easier with AC-130...shoot inside windows, alongside the highways. Invasion and Highrise too. Easy to shoot a big 105 shot into the entrance of a room or window and take out the inside part of that building. Too bad it's not like BF1943 and some of the walls could be destroyed, removing cover. Maps too small for that though, because with the amount of explosives one can use in this game (like grenade launcher and thumper with danger close pro...one of my favorite annoying setups I like to use...especially with scavenger pro).
Really depends on the map. Anyways, I usually have the Chopper Gunner set and will switch to AC-130 on certain maps. Invasion, Underpass, and High Rise being 3 of them I can think of right now. There's a couple others too...like.....Karachi. There's one more.