I've also driven both cars (my cousin has one (GTS-4) and used to own a GTST on Ls)
I found that a normal GTST felt quite a bit quicker, significantly quicker in fact.
But in terms of handling, the GTS-4 had better cornering capabilities, better grip, delivered power to the ground better, launches heaps better........and in the wet? Game over.
I suppose that in terms of modifcations, the GTS-4 would be a better 'base' to work with, with better 0-100, 1/4 mile times.......on the track, it would come down to suspension setup, all things equal, the GTS-4 should come out trumps
Although the GTS-4 handles better than a GTST, as you have said, you cannot compare it to a GTR! Different league altogether.