I can't speak much to the G37 as an owner, but my mom had one and loved it. It seemed lightyears above the G35 in terms of build quality to me.
I have a 2004 G35 Sedan, and it has about 140K miles on it. It is not aging well. The VQ35DE starts hemorraging oil all over the place around 100k, and one of the main culprits are the plastic valve covers that warp. This leads to spark plug tube seal failures and the base gasket leaking all over the exhaust manifolds. Since they made the decision to build the tube seals into the valve covers, a $3 repair just became a $300 repair (per side). You have to pull the intake plenum to get to them, so you need that gasket set as well.
Aside from that problem, the interior hasn't been holding up well either. One of the leather inserts on the rear door started to separate about 30k miles ago; i just pulled it off. The steering wheel has a lot of sun deterioration on it, and some of the weather strips in the door jambs seem to be making a lot of wind noise. This car seems to eat brakes, and I've had to replace the front inner control arm bushings due to failure. Though admittedly those both may just be my wife's driving.