Preemptive cuts out some of the fat on the execution..However, it seems to affect other aspects of the Z, and may negatively impact modules. Some have seen success, and others have not. A clean implementation of preemptive should theoretically not affect the rest of the machine...but the current patches floating around seem to have a negative impact on certain aspects (USB, crashes).
Overclocking will enhance the performance of you Z, but you will be potentially exposing the processor to high heat without the ability to dissipate that heat. This is not only bad for the processor, but also bad for other components in the Z. Not to mention the hit on your battery...I doubt you would squeeze much time using it under such a load. I would say the overclock is much more risky from a physical perspective.
Be careful.
