Yeah this thread is beaten to death...
the fact is that, from a theoretical standpoint, a turbo is and always will be better because the power input required to spin the turbine and connected compressor wheel comes from the exhaust stream, hence the "free power"... A supercharger requires a belt drive to spin it's compressor, making it less efficient...

That being said, it really comes down to what you want...
IMO though, because it is difficult to cool the compressed air down with a supercharger, (a temp increase is inevitable when air is compressed) and it is easier with a turbo, because you can use an air/air intercooler, placed in the air stream at the front of the car, (air/air is also more efficient than a water/air intercooler on Vortech and the likes), it is better to use a turbocharger...

One reason why I chose to go turbo is because of the many options you have available...
- Boost control is far easier than replacing pulleys
- ability to significantly lower the charged air
- etc...