Like he said, the colder the better for each system. When you compress air it gets hot. I think roots type blowers heat soak more than a turbo becuase they are usually mounted right on the engine. There are other differences as well between how they perform. A roots type blower (like on the SRT) give great power and torque from idle up and feel like you have a larger engine under the hood. Turbos and centrifigul super chagers (like Votec) can give better top end preformance and can usually show higher HP numbers. For normal street driving I prefer a super charger since it is instant and works better in the lower RPM range and produces better low end torque. To get the best of both worlds I have seen people twin charge their engines with a roots style s/c and turbo or two turbos of different sizes.