The G30 ///M550 is a great car, and from the Driver's seat is almost identical to the G12. There are minor difference, but none worth noting.
There are attributes that make the G12 more refined. The road noise is greatly reduced if the G12 is outfitted with acoustic glass (not sure I've seen one w/o it, but I haven't checked every G12 in the U.S.A.). The rear seat is a great place to sit, with plenty of leg room, very comfortable, and very quiet - more so than the G30. I can listen to the radio at a noticeable lower level in the the G12 over the G30, and the conditions and situations that you hear the engine (or not) are different between platforms, and prefer the G12, where it is absolutely quiet during "normal" driving.
The door brakes on the G12 are magnetic and allows the door to "hold" in any open position, and for some reason (maybe due to door weight?) are not on the G07 X7's or the G30 (could it be mod'ed?).
The 4-wheel steering on the G12 is the best feature, and wouldn't want a large vehicle w/o it.
Two last things to consider.
- The G12 is so hard to gauge the speed in and must be monitored more frequently using the speedometer. You swear that you are only doing 60 MPH, only to find you are doing well over 80 MPH (dangerous, but the G12 is that quiet and road compliant), where in the G30, you sort of know when you are going "fast".
- The G12 is a long vehicle, so if your garage can't support, plan to park it outside. My garage (house) was built in 2013, and I only have 1" to spare between the rear bumper and the garage door (requires wheel stops to get the car perfectly parked in the garage - and it took the wife a bit to get it parked consistently in the garage)
As a G12 owner, it would be hard to own a G30 platform, and similar, IMHO, between the F10 vs F02, the F02 wins in every way.
Then there is the next generation issue, which is going to be problem as the G70 is just not my style, but the G60 still looks similar enough to the G30....but that's tomorrow's problem.