[Bruce Wayne]

The only cracked rotors i have had (twice) have been on solid/blank rotors. One on an Alfa which gained a significant crack from the outside egde to about an inch. the other on a Range Rover.

Solid, unvented discs/rotors dont dissipate heat so well and heavy driving can cause the rotor to distort. also can happen after heavy breaking and holding on the brake as the pad to rotor contact will give different heat dissipation to the rest of the rotor.

Personally , I would go with OEM or spec of better. The Boxster has cross drilled venting on the 3.2 and I've never had a complaint with the brakes or the braking pad and disc wear has been good too. I see no reason to adjust what works.

On the later range rovers, they are vented but slotted and cross drilled are not a good way to go as an off roader, they can trap dirct, grit and debris, scoring the rotor/surface and reducing braking effect. I had a BMW 325IC once that brakes were poor on, i changed them out for brembo cross drilled vented and slotted with a slightly softer pad it stopped on a dime and gave great hot performance.

there is no one size fits all on solid/blank/vented/cross drilled/ grooved it really depends on the car, but with brakes for road / hard road use, OEM spec does the trick. if it didn't they would be opening themselves up for all kinds of liability suits.