One of the regular posters on this porum, Razzor, provided this list of items that are included in the G4S kit, but not in the G4S RTR:
"The G4S competition kit has additional K factory hop ups as std ie:
UFO clutch ,rear ally bulkhead ,c/f shock towers ,ally wheel hexes ,c/f radio tray ,front oneway/spool assembly ,front universal shafts ,larger reciever battery pack ,ally front and rear stabilizer"
I think that pretty much covers the difference. BTW, all of those parts are available as separate items if you wanted to upgrade at a later date.