I've owned both, though the Infiniti was a sedan rather than a coupe. I did have occasion to rent an Altima coupe. A major day to day difference would be the size of the doors and ease of entry/exit necessitated by the need for access to the rear seat. This is something that could be verified on the dealers lot. Also check out the ease of getting into/out of the seats with the side bolsters.
If you drive the twisties a lot, the Z would give you more satisfaction. If you have occasion to carry more than one passenger, the G gets the nod. The G suspension is tuned for a comfortable ride while the Z is performance tuned. As you realize already, the Z weighs in about the same and acceleration is comparable.
If you want to customize the drive train or body, get the Z.
If you want a car that will put a smile on your face commuting, get the G.