Cars are NEVER 'investments'.
I think it's short sighted to say... "I'll buy an S15 as it's a better long term investment".
Gimme a break. Go buy a house if you want to invest.
You buy a newer, more expensive car, you lose more money on resale.
The bottom line is that the s13/14/15 or r32/33/34 are all COMMON cheap RWD cars.
They have no collectability, they're all reasonable expensive to run, maintain and they'll all tank to some degree on resale IF you can find a buyer.
I enjoy it for what it is - a fast, RWD car. Poor resale means that I can afford a car like this. It's all about bang for buck and IMO the Skyline has it.
If you want something rare to a degree, get a V-Spec GT-R.
T.