the 18ss kit has slightly less top end, same acceleration.
the g3.0 comes with a radio while the 18ss has some chassis hop-ups instead: re-enforced diff's, some other small stuff.

18ss comes with a clear body while the g3.0 is painted (i think).

if you need a radio or are completely new to rc nitro) then the g3.0 is the way to go as it is assembled and ready to go.
if you have a radio allready and are not afraid of turning a box of parts into a nitro car then the 18ss+ is the way to go.