I owned a c4 for five years. Thus I cannot speak from personal experience. However, I have read every single thread forum nazi put up(because I was thinking of buying a C5).Thus I have a good feel for the differences.
The C5 defeats the NSX in every performance category. The margins are not enormous stock but they are there. Furthermore, the C5 can be upgraded to higher HP ratings for very little money.
From what I understand, the NSX is the most refined sports car in the world. It has supercar performance, and exotic styling with Honda Civic reliabilty.
The above is merely the short and skinny of what i have read. Perhaps it is too simplified.
Either though, I would weigh what I mentioned above and really determine what you want out of a sports car. Ask yourself what are you going to do with it. After you determine this, then you can get in to the cost issues and more detailed aspects of choosing a sports car.