IMO the S30 line is the best Datsun/Nissan has ever designed. I'm into creature comforts and believe the last year production cars of a certain line are the utmost expression and refinement of that specific model. Since most buyers go for the early S30s, the law of supply and demand will allow you to buy a 280Z for less money than a similar condition 240Z. You'll spend less money and end up with the a car whose "bugs were worked out" by Nissan (great AC, reliable EFI, decent electronics, good suspension, decent wipers, etc.) and can be daily drivers without any sacrifice AND still enjoy those gorgeous lines. Eventually all S30s in comparable condition will have similar value so take advantage of the bargain, less respected 280s at this time. Other than the extreme purists, most people won't care if you're driving a 240, 260 or 280. They will just get a kick from seeing an S30 on the road and congratulate you.
PS: I hope I don't offend any of the forum friends.