there are 2 main types of superchargers on the market...the roots type (jackson racing and mini cooper s) is a much more subtle unit that gives you the low end torque that most people think of when talking about a s/c

another type is the centrifugal unit like in vortech...these give gains near what a turbocharger can give and act similar...you need boost to feel power and sometimes you get the lag associated with these units

smaller turbos spool faster giving more low end power but do not help at the high end...take the porsche twin turbo...it runs a set of 2 turbos...a smaller unit for the better response and a larger unit to give the high end gains...both are tuned to create a good torque curve so it doesn't have a dropoff in the middle while the small turbo gives up and the larger unit isnt' fully spooled yet

the reason why s/c are better for emissions than a turbo system is b/c of they do not require exhaust to turn its compressor (belt driven)...turbos increase chance of exhaust leakage and cool the exhaust gases down requiring extra cat converters to be added inorder to properly clean the exhaust (the cats work best with high temps...that is the prime reason why hondas have moved most of its cats onto the header so it works faster for cold starts and better when heated up)