87's have speed density fuel injection instead of mass air flow fuel injection. They made a LITTLE more power in stock form (probably less than 5), but do not respond well to engine mods. You can convert them to mass air fuel injection for under $500 I think. Not exactly sure on the price, but it's not too difficult.
87's were available with T-tops, which the later ones weren't.
I beleive '87 non-GT 5.0's had the 4cyl seats, not the GT seats that came standard in all later (90+?) 5.0's.
Basically they are the same car. Same engine except the fuel injection, same tranny, rear end, and brakes. If you have been looking at 88+ cars, there's no reason not to look at 87's.
Anyone feel free to correct me if anything I said is wrong.
Jeff