The only AWD system I'd even consider for racing is that of a Skyline's, which I believe has a continuously-variable power transfer rate, based on rate of accleration and other various conditions.

I remember it being described as this, by some magazine.. of course, I'm likely wrong, so if a skyline-guru could clarify, that'd be great,

Full throttle-
Back tires light up instantly. Computer (?) realizes that there is zero grip, and transfers just enough (up to 40%) to the front wheels to get it moving. This starts the car moving, and at the same time, allows the back tires to grip. Power is then re-routed almost fully to the back tires, but just enough remain with the front to keep from spinning the back.
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Other than that, I'd consider AWD for autoX, but for dragging... RWD all the way.