I would say one or the other, not both. Either drilled or slotted, not a combination. If you think about it, there's actually less surface area on a drilled and slotted rotor than on a solid rotor and while the difference may be neglegible, it still remains present. More so with towing or high performance braking, it wouldn't bother most people.
I like slotted rotors myself. They clean the pads better and from what I've seen aren't near as prone to cracking as are drilled rotors. I also prefer drums in the rear on a tow vehicle (if that's what it came with stock). They work fine and a 13" drum with a 3" wide pad on the 14 bolt under my truck will stop pretty dang good. That's a lotttt of surface area.
Discs work great on the rear too, but unless there's something majorly wrong with your drums out back, I'd leave them there.
