I am assuming that you are trying to decide between the kennebell 2.1 (or 2.6) and ANY centrifugal, because you didn't specify which vortech you wanted to use. Vortech, Paxton, and Procharger make different models of superchargers that can cover everything from 350-1200 horsepower in completely streetable kits.
You need to decide what your supercharger needs are. Are you going to do all your racing at the track, the traffic light, open road, from a roll, from a dig, etc. Do you want your car to have big-block torque down low and all through the RPM band, or build power more and more as the RPM's climb. Also, the max amount of boost you want to make can play a big part as well. Lets not forget there are plenty of nitrous kits out there that can give you the same kick for roughly a third of the price. Then for your consideration, there is the freakish power that the 4.6 is capable of with turbos.
These are important issues and making a decision will affect your future mods such as rear end gear ratios or camshaft selection, and dictate whether or not you need certain parts (specifically an intake manifold for centrifugal blowers, turbos, and nitrous). Really there are a ton of decisions to make depending on which route you take.
There is also the build-up associated with the poweradder: fuel systems, forged rotating assembly, and strengthened driveline are necessary to make the most of your setup.
Bottom line, you can (and will) make serious power regardless of which route you go. The question is when and where you want to make it, how you want to make it, how much you want to make, and maybe most importantly - how much you are willing to spend.
Sorry, back to the question...the quick and ugly version is that in kit form the 2.1 is going to closely match up to the Vortech V-2 but be worlds apart at the same time.
Think of a twin screw like a baseball bat and a centrifugal like a rapier. One uses brute force by utilizing a large amount of torque all the way to torque peak. The centrifugal is much more a technical exhibition, building more and more boost as RPM's climb. Both achieving the same goal of going fast quickly.
Assuming you run 10-12 pounds of boost, you will make more torque throughout most of the rpm range with the twinscrew. The Vortech may make slightly more peak power, but it will lose out big time on low end torque. That is not necessarily a bad thing because more than likely, from a dig the twinscrew will make it a challenge to not overwhelm your tires. Once into the powerband in second, and especially beyond, the torque issue is moot if you are properly geared. If you have the skills, you would be better off on the street with a twin screw. On a track...well, there is a reason that centrifugals dominate the track. Using slicks, and dumping the clutch allows you to pretty much bypass the low end of the rpm range and move straight into the powerband.
All other things being equal, in a quarter mile, the twin screw will hold a definite lead through the eighth, but the centrifugal may be able to overcome the gap by the time they hit the traps. You may get other opinions that argue otherwise on this though, who's to say who is right without actually lining them up.
I hope this helps, but if you can give me more specifics, I can give you a better answer.
