I don't pretend to know alot, But I beleive that there are several differences, but the most major one is that the GTS only has a single turbo, I think the GTR's have twin turbos, But again I am sure some of these GTR boffins will put me right.
Depending on the model type (R32/33/34) the spec varies hugely.
Firstly lets handle the GTR,
It has a 2.6l twin turbo engine, Rear wheel steering (HICAS) and 4WD (ATTESSA), and weighs about 1500Kg
Then the GTS,
It can come with either of the following engines, 2l non turbo, 2l single turbo, 2.5l non-turbo, 2.5l single turbo, some have HICAS and the 2dr coupe weighs about 1370kg. There are 4 door variants, and some 4wd versions with the 2l engine.
There are more differences than this, but should be enough to get you going.
A well spec'd and very clean GTS25T with low miles should collect in the region of 10-12k.
There are loads of low spec, high mile, poor quality cars available round about the 6-8k mark. Same can be said for the GTR with some cars collecting really low prices, you get what you pay for.
Some other differences,
The GTR has some really tasty race seats, the GTS seats are more high class recliners, but still a good seat. Many body parts are common to both cars, and the suspension setups are very similar too.
This can also be said of the cabin trim, and it's commmon to replace the interior with GTR parts, a full set of seats and door cards would just about complete the interior conversion for the un-informed.
The GTR makes a conservative 280bhp form its 2.6l, whilst the GTS makes a enthusiastic 250bhp from it 2.5l.
The GTS power to weight ratio can make up for a lot, but in no way comes near to the added driveability of the 4wd ATESSA system.
Could have got a GTR but insurance is a bit more, and price was not a real issue when I was looking, but saw the gtst and fell in love really, if there had have been a GTR i guess i would have gone away with the GTR, but there wasn't!
Still on my 2nd GTST and still love it, will never have the GTR full power but is quick enough for me. (for the meanwhile!)
Either way the GTR and GTS in my view are both great cars, have a great past and heritage that only other cars could dream of, and it still makes ppl turn and around a look in awe!
Sure you'd be happy with any skyline choice u make.