As of yet, I have not driven the G35/z or the S2000, but I plan on purchasing within the next year as a daily driver for myself. I have done tons of research along the way ranging f/ car shows, online reviews etc and basically, just need to do some test driving. At this point, the S2000 is much more appealing to me for several reasons,
1. Now that I live in Florida, I want another RWD car.
2. I can enjoy a convertible 95% of the year.
3. Overall, it is a "fun" car in that its quick, good handling, reliable and isn't a killer at the pump. In addition, being a small roadster, it just has a different feel when you sit in it.
4. Priced VERY well for the car you get.

As to having kids, the only thing that bugs me is the lack of Air Bag shutoff on the past models (I'm gonna buy CPO for myself...the wife & kid gets the new car ) but in the end, they use the element or whichever car we get her next anyway.