Here is basically a cut and paste from a PM I just sent.
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1. Vortech/ Paxton vs. Kenne Bell
Well, it depends on what you're building the car for. If your main goal is to build the fastest car in a straight line, the Vortech/Paxton is by far the greater choice. You can build something fast for cheaper, and it's not going to be too difficult to tune. Another bonus is that their customer service isn't as bad as Kenne Bell.
If your goal is a fun street car, or if you're considering Auto-X, road racing, I would lean toward the Kenne Bell. Lots of low-end torque, and on the street, that's what I consider fun. On the track (auto-x, road race), the Kenne Bell is nice, because you have such a wide powerband. The blower is ALWAYS at full boost (above 1800 rpm), so you can use more of the gear without having to shift as much. It has the fun ability to throw your passanger back into his/ her seat at ANY time. That's why I love mine. Here is my dyno curve, this should tell you more about the blower.

One disadvantage for the KB is even with if you have stock rear end gears, most street tires are not going to get any traction.
