APU failure during takeoff roll is a very remote possibility and should not play any part in the decision to go packs off or APU bleed on.
Having flown A1 powered Airbi and needing that extra thrust we always go APU bleed on when needed. Actually we use it more on the 321.
Normal preference is FLEX, then TOGA, then APU bleed on. Packs off is only used when APU is on MEL. There is also a certain level of passenger discomfort with an unpressurized T/O, which is what a packs off condition gives you. Now I will admit that there have been a few done when the F/O has inadvertently shut the APU down on the afterstart flow before checking the numbers.