Should I use regular or sp aa

AA will give a bit more bang for the buck in terms of air defense, but they are slower. If you are hurting for food, sp aa could be an option. Hurting for oil (or iron to some extent), definitely go AA. If you are interested in defense against heavy armor than definitely AA.

Thanks

What are you defending from air attack?

Slow-moving foot units like infantry, artillery, rocket artillery, use AA.

Fast-moving mechanized forces like light/medium tanks and motorized infantry/artillery, use SPAA which can more or less keep up with these units.