Between the two there really isn't any major difference in reliability and overall performance...but the FPS difference may be the selling point.
Do you attend fields with designated FPS limits? If so, the A&K may be too hot.
I've had great experiences with both brands. My all-time favorite AEG was my A&K SR-25...which more than likely has the same gearbox as the one you're looking at.
Keep in mind that the G4 series at ASGI are all limited-manufacture...it may very well be that the G4-A3 will remain out of stock. Email em and get an opinion.