having experience with all 3....

i thought the scorpion has poor build quality & heard also heard they are too restrictive.

have a 4" mongoose on mine & its perfect apart from the fact you need major ICE to drown it out on motorway driving!!

the 3" magnex on the S2 sounds the nuts on full boost & can also idle nice and quiet when you want it to.

depends what your after - i chose mongoose cos i thought it was the best all rounder at the time!