the point behind both is to get hot air away from the rotors and allow colder air in, the cooler that area is, the better your brakes will work and the less fade they will experience
imo both generally do the same thing but I have heard people claim the cross drilled rotors are more prone to cracking, I have cross drilled ones on my A4 and slotted on the VX,
personally, I don't believe any of us will be taking our VX on track anytime soon (I may be way off base on that but it seems like more people off-road them than track them) so I doubt you will have to worry about the potential problem with the cross drilled rotors,
the ones on my A4 have held up nicely but I only do 1-2 track events a year so that's hardly a lot of abuse, if you planned on going to a lot of track events I probably would stay away from the cross drilled ones but for me, it came down to price and availability, I don't think either will wear out anytime soon for my purposes