Getting FAIP'd in general apparently closes a few doors (Weapons School for example, or at least makes it more difficult), but it seems most of the T-6 FAIP's who flew T-38s in UPT are heading to fighters now. As for T-38 vs T-6 FAIP, as with anything it is a bit of luck and timing as well. There could be a dude who would be a great FAIP, but there isn't a slot for T-38 PIT available to the Sq at the time, so they end up in the T-6 instead.