Clean&Saaber,
I think that Matt is right; the SPG only came in 3 door hatchbacks. But the molding kits were availiable from the factory as an option and many people scavenged parts from salvage SPGs to get the desiralbe "look."
Google Saab SPG and you'll find some websites with detailed info about the cars.
Speaking of which, if you have a car with SPG moldings and wheels, you potentially have a virtual gold mine, since they're very hard to come by. I don't know the condition of either car, but the Turbo / SPG would definitely be the more desirable car to restore in my book. Unless it's really beyond the pale, consider restoring the it instead of the 900S.
Or get your top, SPG moldings, wheels, and the turbo off the 88 and move them all over to your 91. (I think that Saab changed wheel bolt parttern in 88 and the wheels will fit your 91, but I'm not sure. I know that my 87 900S wheels won't fit on my 91 900 turbo convertible. So you better check that.) Since the 88 apparently isn't really an SPG anyway, there's nothing really lost by parting out the 88 SPG wannabee.
Sc