Center diff is all but impossible on the gorillamaxx chassis- requires a totally custom built unit that also limits thew gearing available due to to the limited space between the chassis rails- a center diff bolted ontop of the motor mount area is far from ideal unless the rails are hacked about a bit (someone been studying the Candyman's truck perhaps?). I'd say stick to a normal tranny as it makes life much easier; you can get a centerdiff to fit inside the stock 3905 and 3906 trannies with a little work....
As for revo platform vs maxx, case of personal preference as to whether you want the revos handling or the maxxs truck style more. Horses for courses....
as Neil said, it is pretty difficult to get a center diff to fit on the stock Gmaxx chassis. You'd have to get pretty creative to get one to fit. I'm cooking something up at the moment, but i think im going to have to notch the chassis rails a little, among other things, so yeh... it'll get pretty 'custom'
Thanks for al the help guys. I've decided to buy an E-Revo upgrade it a bit and then transplant it all into a G3R chassis. Oh and Im sure I'll be able to fit a centre diff in somehow .