The differences between a GT5 and a GT5-S are purely cosmetic.
A GT5 is essentially a stock bodied Pantera with fiberglass fender flares glued on. Between the flares and over the rocker panel is a 'running board' ground effect. In front is an 'air dam' type of spoiler.
A GT5-S is basically a re-bodied Pantera, meaning the fender flares are part of the fender/quarter panel stamping, done in steel (it is said that the 'S' stands for steel). There is nothing linking the flares together over the rocker panel. In front is a 'chin spoiler'.
Other than these exterior differences, the two cars are identical. GT5 bodied cars were available from the factory from 1980 to 1989 or 90. GT5-S bodied cars were available from the factory from 1985 to 1990. Both could be had either with or without the delta wing.
Here is a link to the P.I. 'Cars For Sale' page with both of these examples listed for sale. The GT5 is the white one on top, and the GT5-S is the yellow one right below it.
P.I. Cars For SaleThese both look like very nice examples. There are fans of the GT5 look, and there are fans of the 5-S look. It all comes down to personal preference. Good luck with your search, I'm sure you'll be happy either way!
Michael