The Veyron is understeery as hell, too heavy for it's own good, despite that, it can get around if you're patient enough with it.
FXX is pure Trackcar, like the Zonda R, it's brutal yet more than capable of lapping anything.
Both require patience though.
As a dragcar? I would say the more powerful of the two, but the FXX with tuning can match the Veyron well enough I guess.