It's all about design and quality. Personally, I think the 370Z interior is a mess of materials and the mixture of digital and analogue is jarring, and in that respect I find the 350Z much better. Clearly the finish quality is better on the later car, though.
On the other hand, whilst the 9x7 cars (assuming you mean that age, judging by the cost) may look a little dull, it's incredibly functional and the quality is bang-on the money. A 100k miler 9x7 interior will look as good as it did the day it rolled out of the factory: Try saying that with a Ford or Nissan. the 9x7.1 cars look dated due to the PCM 2.1 system, but the 9x7.2 cars did improve on this with the PCM3 touchscreen (part of the reason I wanted a 997.2 car!). The 9x6 cars were a shambles, both in quality and aesthetics, but if you go back to the 993 and previous you'll again see that even 30-40 year old cars are holding up superbly, and much better than anything else of that era.
Try finding a 355 without sticky buttons or peeling leather and plastics, or trying to find everything first time on a 458. The first is poor quality, the latter poor ergonomics. And they're much more expensive than a 370Z!