Slotted is used in racing applications to improve cooling. Slotted replaced drilled there a long time ago; Google the history of drilling rotors and their original purpose, and you'll understand why drilled is obsolete today.
That said, slotted for street isn't rational IMO; you can use them but they're not as quiet, eat pads faster and are more expensive. OEM blanks are dependable, work well in all applications and are economical. Frequent track goers including myself use blanks for all of those reasons, and the fact that they last a long time. Unless you're concerned with appearance, in which case our brakes look crappy anyway, blanks are the practical route.
2011 BMW 335i E92 - Le Mans Blue, 6MT, M Sport. JRZ RS Pro, Quaife LSD, BMW Perf Exhaust