crossdrilled is more for looks i think......but you can always get rotors that have both slots and xdrilled on them.....Rotora has that....

 

As dklau33 x-drilled that are cast are stronger. But, even drilled rotors can be designed with some integrity. Properly choosing the locations for the holes and then radiusing the holes is important to reduce the chances of cracking. I used 2 sets of cross-drilled rotors from Stillen on my 240sx and tracked both sets. The rotors held fine. I don't think they saw the temperatures we might though since they were 300ZX rotors which have a lot more mass than the stock 240 rotors.

And I noticed no excess wear on the pads.