Everyone knows about modern brake pads no longer de-gassing and the added stress risers from the holes, so drilled rotors are now out of fashion. Slotted rotors can serve one small function in racing: to scrape away a tiny amount of pad material, to keep the pad surface "fresh". If your pads are prone to glazing this might be advantageous, and many 2-piece racing rotors are slotted.
http://youtu.be/_L_ev1iuGzg
That video has a lot of marketing fluff, and honestly... I think drilled and slotted rotors is still mostly about image. There is no cost effective 2-piece 14" Mustang rotor (they cost 5-10 times what a 1-piece rotor does) and the slotting/drilling is mostly unnecessary, in my humble opinion. So on our Mustangs with 14" front rotors we use Centric premium rotors.
Best bang for the buck. They have a coated center so they don't rust the first time it rains and they last a good long time.
But if you like bling, go for it. We just put slotted and drilled rotors (and upsized both the front and rears) on our shop truck. Because I'm secretly a pimp.
At least the rotor manufacturers are now using a generous chamfer on the holes and ball nose endmills for the slots. So it isn't as rotor cracking as it could be...
