eggman, i havent read all the posts here, it was getting rather opinionated and heated, so ill just post my experiences.
4 years ago i was in the same boat you are in now. i drove a 300zxtt stage IV and was looking to move ...well..... up.
I say it like this because the 300zxtt once you get to stage III is absurdly fast and an all around amazing car.
my choices at that time were an F car (328, 348 or mondial were what i was focusing on). I subscribed to both the ferrari list and the NSX list and sat back and listened for several months, and once i felt i was somewhat educated, i asked some questions.
my main concern was i wanted something that was high performance, somewhat unique, somewhat prestigious and at least as reliable as the 300zx was (though mine was starting to show its age and wasn't so reliable anymore). my price range was around $40k, give or take a few k.
it really came down to did i want ultra prestiege with huge repair bills (as research from the F car list showed me that pretty much any ferrari came with huge repair bills and i absolutely should EXPECT a lot of problems, both small and medium with any F car) or did i want something that was still exotic, uncommon and as reliable as my dads prelude. I also had to remember, NEITHER of these cars had much more than 1/2 the HP or torque of the modded 300zx.
common sense won over which is unusual for me
(mostly because money was a large concern) and I went with the NSX (and kept the 300zx). generally i put around 20-25k miles per year on a car and the 300zx was no exception. after i bought the NSX, I didnt put more than 1,000 miles on the 300zx in over two years. the 300zx felt WAAAAAAY more powerful, but i took both cars to the drags and they ran almost identical times (as i recall it was 13.5@109 for the 300, 13.6@108 for the nsx with headers, airbox, exhaust, no cats, ie about the same cost as a stage 3 setup on the Z). as you have a 300zx, you're aware it's difficult to put all that HP down to the pavement.
i finally got rid of the 300zx last year and while it was a great car and i sometimes miss it, i know i made the right decision. im a car guy, i love cars, i love driving, i love to hop into a car and just go, maybe for the afternoon, maybe for the weekend, maybe for a couple of weeks, spur of the moment, and not worry about reliability and whether or not the car is going to be any sort of concern.
maybe someday an F car will be in my future, but it will be at a time when i know i can afford the repairs that go along with owning one, or the day when F gets off their asses and makes reliability a priority (which will probably be never).
in the meantime, the nsx engine sounds amazing (never occurred to me with the 300zx that i'd find an engine sound an enjoyable driving experience), the high red line is a blast, and the twisties are much more fun. the only thing i miss about the 300zx was the mega gobs of torque when those turbos kicked in and you were SLAMMED back into the seats all the way from 3k to redline in every gear. funny that the performance numbers ended up being so similar between the two yet the feel so different.