You can also get electronic climate control in an '86.
1987's have shown to have more resale value because there are more "zero mile cars" that people bought as an "investment" and put away. You couldn't do that with an '86.
When the '86's first went on sale, nobody outside the relatively small Buick community really believed they were anything special. The Car and Driver article where one ran 13.9 stock didn't come out until April of '86.
By then, the '86's were about gone, and you had to order an '87 (and dealers "adjusted" the stickers of the '86's on the lot up).
There is zero reason for an '86 with the same miles/condition to be worth less than a '87 from a collectors standpoint. It was, after all, the "original" Intercooled GN and the one that cut the 13 second 1/4's in the magazines.