Slots are to help remove water more quickly via the slots.
Drilled are for extra cooling but in my experience (won't buy drilled again) they make a low frequency drone sound and I can feel it in the pedal too. Just an odd feeling.
If you want better cooling look for angled vanes as opposed to the standard vanes which go straight out from center outward. Angled vane rotors are side specific so they point foward to grab air as they turn forward pushing it inward cooling more effectively than standard vanes. Extra cooling is sometimes needs with mountain/hill passes, towing, stop/go, ect.