Cross-drilled/slotted are for looks only. Most CD/S rotors you see are actually physically drilled, if you plan to do any real hard braking such as in a track event they will crack much faster than a normal blank disc.

Slots are no longer needed because modern pad compounds do no "gas out" like older ones do... the slots gave the gas somewhere to go so the pad could maintain better contact with the rotor.

Cars like Porsche and Ferrari have rotors where the holes are actually cast into the rotor during creation, not drilled, thus they are marginally better.

When I track my car I run blank rotors from NAPA...