No conundrum no regrets. The sprints I used to run in were not for sheep stations, but I liked getting the most out of the GTSt. My plan is to move it on and have a dedicated, unregistered race car but there are no reasons that a GTSt won't be as fast as a GTR on the track. What I constantly see are GTR's with the same mods, the same lack of proper setup and so on and similar lap times to each other, and yet the couple that are well set up are a serious margin faster again. The GTR is simply a better starting point for a novice, but ultimately would be slower around the track due to weight with a decent setup. The GTSt needs stiffening, suspension, responsive power, diff, and when we get this fast possibly greater braking capacity, but it will corner so fast less braking is needed. I'm just not willing to do this to my perfectly registerable GTSt, so will be letting it go for a new race car.
Otherwise it's all personal choice. I like every one of the 5 cars I have, and need good reason to part with any be it RWD, FWD or AWD.