You can check the roadster section and see if there are any members that mention any issues that come up.
Nissan 370Z Roadster (Convertible) - Nissan 370Z Forum
I'd imagine with a soft top that there will be issues with the top, closing/opening feature thingy, leaking, etc. But all can be maintained if you know what you're doing. I honestly don't know since I have the coupe version.
Coupes are lighter than the roadster, so I guess people get rid of the vert and go with the coupe to save a few pounds? or maybe because of additional family members, practicality, etc...I know if/when my parents aren't able to drive themselves anymore or when I start a family, I'll be moving onto a sedan