it doesn't matter whether it was a stagea or R33 IF money was no object i would choose the 2835 Pro S over the GTRS.
however, the pricing you are quoting isn't quite correct. (i think you are quoting the turbo on its own)
the GTRS kit from nengun is $2545.64 and bolts straight up to your dump pipe and has all lines, hoses etc. the factory heat shield can be modified to fit pretty easily.
the HKS 2835 Pro S kit delivered is $3234.78 including all water and oil lines, a new heat shield (from memory) and the HKS short split dump pipe (turbo won't bolt up to exhaust without it) which may mean you will need to get a new front pipe or modify your existing one.
the difference in price isn't quite $1k (apologies the price difference has narrowed a lot since i last looked) but it might need a little more fiddling on the exhaust side.
on the correctly specced GT3071 i mean one with a 60mm turbine. listen to the mafia on GT3076 selection.