Both cars are old models however the G4 is upgradeable to the spec of the G4s by buying a upgrade kit.The Mtx3 however is not.
Both cars are good cars but as said before are not current models.
Look at parts support in your area and look at getting current models.
Are you looking to race or to mess around ?
If looking to race ,your choices are :
casterracing EX-1 Pro, SP Silver Arrow
TOP Racing Scythe, SP 17.5, SP GT Pro 2.0
TOP Racing Scythe, SP 4.5 V2, Hobbywing Xerun 120SD 2.0
TOP Racing Scythe, SP 13.5, Hobbywing Xerun 120SD 2.0