As Phil said with the engine changes - hardly seems worth it as I'd be surprised anyone would notice such a tiny difference. Gearbox software is different on the GS to all 4200s AFAIK
The gearbox software perception intrigued me. So after a bit of digging it appear that the GS and facelift 4200 had the same gearbox actuator upgrade, software upgrade and flash module upgrade as follows :-
HW CFC 201 (SOFAST) up to assembly 12203
HW CFC 231 (SOFASTII) from assembly 12204
(Taken from the maserati advanced electronics manual)
The perceived faster gear changes seem to be as a result of the auto press comparing pre facelift 4200 and GS's or possibly a marketing ploy by Maserati as the GS was as far as i know launched before the facelift 4200 and if that was the case then the GS would of had faster gear changes compared to the 4200 at the time.
As a complete package i agree the GS is probably a better car but it really comes down to personel choice as to which interior you prefer and if you prefer the more aggressive appearance on the M138 body.
I'm sure Voicey or Marios would be able to give a definative answer regarding the gearbox software.