The GTS-T is a much lighter car than the GTR, and if you're going to run RWD anyway, then there's little use in carrying the added weight of 4WD. Another thing to bear in mind is the maintenance and repair costs of a GTS-T compared to a GTR. Find out how much for a used RB20 and RB26...
That said, a GTR is a GTR, and for that reason alone, it's always seen as the preferred option. Most people like to own a GTR so they can brag about owning one -- but for actually driving and maintaining a ride (especially in RWD), go the GTS-T.