I would go with a GT, if I had to choose it would be an '89. That way you are mass air, probably have 140 mph speedo, do not have that big irritating steering wheel with the airbag and no tilt. Go here for all of the detailed changes from year to year... http://www.mustanggt.org/91gt.htm
Here are some basic differences and downfalls
1987....... 85 mph speedo (if that matters to you), speed density (really not a whole lot of camshaft choices, and modded the car will hunt for idle a little regardless), t-top option
1988....... same, very, very few changes
1989.......conversion to mass air ( no more speed density, way more choices when modding, but did drop hp a little because of the MAF sensor size), 140 mph speedo (some still had 85mph), no more t-top option
1990.......mass air, addition of air bag, no tilt steering, possibility of no center console, slight drop in hp but this was not reflected in the overall rating
1991.......bigger fender openings for new 16 " wheels another slight drop in hp
1992.......power seats option, basically the same, lowest production though and very hard to find.
1993.......switched to hypereutectic pistons, slight drop in hp again, new rating system drops it to 205hp 275 lbs/ft.
There were slight engine changes in the GT from 87-93, and all of them decreased hp, so basically the earliest will have the most hp, but mass air really is nice when you mod, thats why I like the best of both worlds the 89. I drive an 88 however and its great. Good luck
Cardinal Red '88 GT 5 speed, 99 Contour Electric Dual Fans, Battery relocated, 130 amp alternator, 76mm Mass Air conversion, 75mm BBK throttle body, 5.0L .030 over, BBK SSI, AFR 185cc heads, Comp Cams XE276HR, BBK ceramic longtubes, Spec Stage II clutch, MM front/rear coil overs, MM front LCA's, MM manual brake conversion, Flaming River Manual Rack, 94-04 Cobra front brakes, 99 Cobra IRS, Torsen T2R differential, 3:55 gears