Wing or diffuser for splitter.
 

I currently have an APR splitter which I believe to be functional. Should I go with a wing or a diffuser to balance the aerodynamics out?

Either one will add downforce to the rear. Diffusor would be more stealth and have the least drag for a daily driver but not adjustable for different downforce levels like a wing and probably not able to give as much total downforce either (wing will have a larger Cd hit though...) - assuming you are talking about a functional racing wing and not non-adjustable one.

Diffusor is on my list to do to my FRS...