The pad contact area is less with slotted, even though the surface area is more. That extra (third) dimension doesn't count in the least bit for braking because it isn't used for a contact surface. In fact, there is less contact surface because you've still removed material from the contact plane. You can't add area to a plane by removing area from that plane.

You guys also need to realize that there is a lot more to rotors cracking than if just if they are drilled, etc. The otherall thermal mass of the rotors can be very different depending on how thick they are, how much physical metal is used, and other things like how fast they can transfer heat from one area to another or how fast the heat can be removed through venting, etc. Then you run into vent design like number of vanes, width, thickness of vane webs, etc.
The only way to make a blanket statement about cracking, warping, etc., is with the same rotors, only differing by the cutouts (holes, radial slots, J slots, etc.) If you have a skinny little rotor that's cross drilled, and then go to a vented rotor that's cross drilled, they're going to be different.
There are a lot of very successful drilled designs out there, and I'll bet that if you find a racing class that uses metal (i.e. NOT F1) rotors, and you see them using blank discs, they are doing so because they are limited by class rules to using a max size rotor so they'd rather have the extra surface area (from not having holes).
It really depends on whether you need the extra area or the advantage of holes/slots.