to my knowledge, the Roushcharger is THE only roots style blower for an S197. All of the other positive displacement superchargers are twinscrews. The magnacharger is a roots hybrid(kind of a cross between a roots and twin screw in terms of rotor design). Roots tend to be a little less effecient then twin screws.
I personally would sacrifice the extra top end horsepower of a centri for the instant low end of a positive displacement anyday! It just makes for a car that is more fun to drive. IMO

 

I agree 100% on all points GT4HUTCH!! ^^^^^

 

Whipple FTW! haha IMO here is the rundown:

Twin Screw : Centri

Whipple : Paxton
Saleen (very very close second) : Procharger
Kenne Bell : Vortech

 

Once you determine what type of blower you want than you need to figure out how much power you want from it. Than we can really narrow down your options.