I own both and the URX4 is the best feeling and fitting rail IMO. The MK16 that comes on the URG-I is a much closer comparison to the URX4. The MK8 is shaped like a cross and not the most ergonomic. It forces your thumb to the 10:30 position for a right handed shooter.
But as far as being bomb proof, G rails are absolutely there with KAC.
ETA: astronomical KAC prices? They are about $25 more than G rails