Hey salmaan,
GT=Grand Tourer.
R usually stands for Racing edition
S usually stands for Sports
X is usually optioned spec
i usually means injected
Which is more powerfull well thats pretty hard to tell as General GT class covers all the road sports cars in cluding suzukis (GTi)--- Nissan GTT,GT,GTST,GTR--- Porshes (GT) and ferraries etc.
So every manufaturer makes it according to their market share. and compete only for the class the car is specified for which is normally difrentiated by their engine power.
hope this will help