I think what the 370Z gained in luggage space by repositioning the rear strut bar to directly behind the seats, it lost in cutting inches off the wheelbase and by having a conventional gas tank compared to the 350Z's saddle tank.
The 370Z has 6.9 cubic feet of storage space.
The 350Z has 6.8 cubic feet of storage space (for the coupe, not roadster)
...When I had my 350Z I had to travel only with duffel bags so I could cram them in under the strut bar. Oh well, I learned to live with it as the car was too fun to drive. Getting 4 new tires put on and taken to the tire place was a PITA though since I could only take 2 at a time.