IMO there's not much to upgrade to.
It's either an r33 GT-R or an r34 GT-t sedan.
I do like 4 door sedans to be quite honest but I only like cars like the BMW M5 or 535i or Commodore SS or Falcon XR6T. But they all have their problems. The r34 GT-t satisfies my four door lust. In some ways I like the Stagea too.
I find all non attessa 4wd systems to be just inferior. What can one say about the GT-R???
I think the Silvia is not an upgrade to the Skyline GTS. I find the cabin less impressive, the chassis less impressive, uses the same fuel, same sorta power, mods are slightly cheap... IMO more of the same problems as the Skyline but lesser.
I am fussy about cars and ATM the GTS is it. If you can name something better that's not a GT-R I'd like to hear it.
T.