On a 91 8v you probably have 239mm discs. I may be wrong, but I think some run out cars may have been upped to the 16v brakes, with a 256mm disc. So a quick measure of the existing may be worthwhile just to make sure.
if its 239mm, you need the wider vented discs with the pads to fit. Other non GTi models had 239mm solid discs, with thicker pads in the same calipers. So make sure you get GTi items.
Standard parts from GSF have been good to me, as said above, drilled/grooved discs are not raved about on these cars, but feel free.
If you want more braking power, or suffer fade- upgrading to the bigger 16v or the 280mm G60 brakes is the way to go. Hae a search for all the info, its been covered lots on here.