I've owned a 33 GTS-T and 33 GTR and I was surprised as to how much better than GTR is to drive. I mean, I know it's not a fair comparison, but even cruising around off boost, they feel quite different. The GTR feels a lot more tough and weighed, but doesn't feel that heavy.... certainly feels huge though, due to the lower seating position etc... it feels like a massive car but I love that, you get confidence from it.
GTS-T's are still great cars though, no doubts there
I'd say it's worth the upgrade though. I always thought they'd drive quite similar, I was pretty surprised when they didn't.