Both companies produce a good product. It just depends on what you are looking for. The Magnuson provides boost from just off idle so your feeling that between 2000 and 2500 RPMs. Great for a daily driver! The ATI's boost comes on higher in the RPM range (usually about 3000-3500). This one has greater capibility if your looking to build a big 600hp horsepower motor.
When people ask, I always recommend a positive displacement supercharger for street-driven cars. The power is so much more streetable and a lot more FUN. Having 80% of your boost by 2200 RPM is a great feeling. It's a no-brainer to get the Magnuson in my opinion.