what's better?

 

This question is kinda like asking what your favorite color is on the car. I can justify any of these answers as being the best with a slight modifier (same one used on all of them). If you are looking to upgrade to better brakes on your car, all of these will work provided you find someone that sells them made from decent metals. The cheap steels tend to warp easily.

I have upgraded the rotors on my car and I got some that were cross drilled and slotted. The slots are meant more to allow any gassing (vaporizing the pad material) that occurs during hard braking to be vented, keeping the pad in contact with the rotor more, leading to better stopping power. Granted, the drilled holes will do the same thing. With the drilled holes (whether only through one half of the rotor or all the way through), they are meant to aid in air flow through the rotor, allowing for more heat dissipation, therefore the rotors are cooler after repetitive braking, allowing the pads to last longer and helps to prevent warping of the rotor since it is not getting as hot.

 

First, I agree with Thermo about choices among drilled, slotted, cross-drilled, kind of like splitting hairs based on certain racing needs and matching of different pad compositions on track cars. In general, I don't recommend any of these choices on a street driven car. To gain any technical benefit, the street driving would need to be very aggressive, and I'd expect your license would be gone long before any technical benefit would be realized. Drilled/slotted rotors also shred pads rather than wear them so pad wear is greater to much greater. The upside on a street car is that they look really cool. My choice is all related to aesthetics.

I disagree about rotor warpage. I have yet to see one example of warpage on a modern dual wall rotor. In fact, in 30 years, I've only seen a warped rotor a couple of times on track cars and they were special setup single wall rotors.

The "feeling" of warpage comes from the leaching of resin out of the pad compositions that distributes itself always unevenly onto the rotor surface. This gives the feeling of slip/grap that is often misdiagnosed as warpage. I've proven it every time with dial indicators on the rotors. The resin leaching usaully is associated with cheap pads, or certain pad brands. In the cases I've worked, I've switched pads to ceramics, cleaned up the rotor surface and the warped rotor feeling suddenly goes away.

Heat dissipation is a dual edged sword - The mass in solid steel rotors acts as a heat sink so the extra material works in your advantage. Drilled/slotted rotors, where mass is now missing cannot absorb as much heat, but the drilled affect on a spinning rotor works to dissipate heat a little faster than on solid material. Once again, it takes pretty extreme driving for these factors to start making a difference where tradeoffs can actually be discussed.

Bottom line - I'd not worry about which rotor type is better, just what you like to look at while the car is standing still.