Drilled ones should be counter sunk to prevent cracking. I've not had experience with them really so I don't know how much of a difference it makes.

Grooves are better for letting out the bad gasses and improving surface contact. Drilled are to improve cooling.

Fish-hook slots are the best I believe.

I can't say I've checked my wilwood drilled and grooved discs for cracks ever, but I'll have a look at them next time the wheels are off.

JAmes.