Slotted allows the gasses to escape to the outer diameter of the rotor....reducing the 'air hockey' effect between the pads and rotor. Also works the same way for removing water (usually found in wet weather
). The DRILLED rotors are prone to microcracking between the holes. The other problem, like I stated before, is that you now have less surface area for the pads, and thereby less braking. Brembo sells their rotors under the marketing idea that a cooler rotor doesn't allow brake fade. (the holes are advertised for cooling the rotors, which they probably do a little bit, otherwise Nascar wouldn't do it, right?)
I'm here to tell you the last thing you need is drilled. Buy a good quality hardened SLOTTED rotor, and put ceramic pads on it. They'll NEVER fade. They actually bite harder than cold stock rotors when they're hot. This negates the whole purpose of drilling.
Brembo sells cross-drilled rotors because people like to look at them through their wheels. Supply and demand.