the GTR option would be the better choice. changing a GTS-t to have the GTR's AWD system would be the biggest f*ck around you could possibly imagine. you'd have swap over pretty much the whole suspension system too as having front axels means you need the GTR's setup not to mention the electonics as well!. because they are very different, your budget would definately blow out....
if your want to get more power to the ground for your GTS-t, then get yourself a mechanical LSD, sticky tyres, your suspension setup improved and learn to feed on the power rather than just planting your foot and wheel spinning! its a problem that all GTS-t owners have. even a moderately powered GTR will easily spin all 4 wheels in the dry!