A1 Focus or A1 servo

whats the best mode to shoot with on fast moving subjects?

Servo

AI Servo

AI servo always.

AI focus is simply a setting for when you don't know which to use (one-shot or servo), all it does is decide for itself which mode it thinks you want by defaulting to one-shot then shifting to AI servo if it thinks the subject is moving. This can cause a lag between shifting from one mode to the other and result in shots being taken whilst in the wrong mode.

Examples would be when focusing on a bird about to take off, or any other subject which starts off still and then suddenly moves. You know that you are waiting for the subject to move, the camera doesn't. When you try and get the shot of the bird launching into the air, the camera won't have time to switch to AI servo.

I frequently switch between one shot and AI servo myself, when I decide to. I tried AI focus when I first bought a DSLR but found it just doesn't do a good enough job of decision making, leading to missed shots.

That's almost certainly something they threw in for the beginners with DSLR cameras, as P&S cameras don't let you make the choice.

Besides, it's AI, not A1.

Forget AI-Focus even exist.