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I came from an 06 S2K, and I added bolt-ons and a tune while I owned it.

The 370Z is much more GT. I got the nismo version and, while the suspension is much firmer, the car overall is a lot more quiet and cush. The nismo has a lot better grip and good balance, but it doesn't feel nearly as connected. And the S2000 has very low rotational inertia, which is a lot of fun for canyon roads.

The Z is better as a daily commuter, and the car is less one dimensional. What I mean by that is, the S2000 was flat out crummy to drive in the winter. The Z is just not as fun in the winter. The S is great on canyons and road courses. The Z does that well enough and can also drift, drag, and it's much easier to correct if/when you screw up and get unexpectedly much oversteer. I thought the S2000 was more fun in the canyons, mostly due to the convertible factor, but also due to the lighter weight nimble nature. As an amateur, I'd say it's pretty close on the track as to which is more fun. To track, the Z needs an oil cooler whereas the S is good as is, but for canyon runs the Z would be fine unless you're extremely aggressive.

Relatively speaking, the Z feels like a brute whereas the S felt like it had more finesse. I miss the ragtop in the summer, but like the Z much better in the winter. I miss the nimble handling, but I have great grip, torque, and can slide around a bit without snapping back when the car catches, which can be quite fun.

If you're happy with the S, I would say save your money. If you're ready to move on, the 370Z (nismo at least) is a good option, albeit a slightly different type of car. If you want the same car as your S2000 but better, get a Cayman S or Boxster S.