I would actually think the value of a 97 would be higher than the current value of a 92 for 2 reasons:
1. As stated before, there were a lot more 92s made than 97s.
2. There seems to be a lower limit on how much an NSX will depreciate. For example, I don't think a clean 91 will ever sell for less than $15,000. A comparable 92 will always be worth more than a 91, 93 over 92 etc. A 95+ targa should be worth more than a 94 without, and a 97+ targa should be worth more than the 95. If you pick what you think the lowest a 91 will ever be sold for, then add in for each newer year, targa and newer engine, I would think you would end up with a higher value. (this assumes comparable cars in terms of condition and mileage)
Therefore, IMHO, I think the 97s will be worth more than the current value of a 92. Then again, who knows???