Simple. G37 has rear seats. I can speak to this as I recently flipped my G37S 6mt for my M3. I was in a different financial situation and the car seemed like a good choice at the time. My observations of the car, over the 6 months I owned it (which should tell you something) are as follows :
Not a bad interior, with a better nav/in-dash system than my CCC iDrive (not that that's saying all that much)
Engine was OK actually... Sure, it was a bit rough, but pretty torquey and flexible. I started to enjoy the exhaust note as well.
Handling... Not wonderful. Nose heavy, pushed in corners, but could be provoked into oversteer fairly easily.
On the track I managed to hit limp-mode in under 3 minutes. Not impressed. As I understand this is not unfamiliar territory for this engine even in a 370Z.
Brakes were good, but I expected more from the big monobloc calipers.
Overall, a solid car for daily driving. Not terribly exciting and NOT suited to a track, even if just on occassion.
Now, having said all that, my M3 is 10 times the car the G will ever be. Its much more direct, nimble, and clearly much quicker through any measure. Better feel from the steering wheel, brakes and clutch (the clutch on the G was very on/off and a bit annoying). Shifter isn't as good as the G, but I'll take that shortcoming any day.
Just my .02... I'm expecting change.