From what I have seen of the P series chassis, they are cooler running, as it is a bit bigger, and also has thing ventilated separately. The CPU intake comes in the front right and goes out the front left, while the hard drives are cooled through the top front to back and the rest of the system gets cooled front to back from lower in the case. Where as in the G series, it is all front to back, including all the heat off the CPU heatsink.

Noise I can't really judge as my only experience with the P series was at Quakecon filled with tons of other noise. My personal preference in systems has moved on from Shuttles to notebooks or tiny Macs.