The GS would be simpler to do and will look like most others unless you are really creative.
The Xv however would take more work to get where you want it, would be incredibly unique in a crowd of CB-wanna-be's.
If it were me I would go with the XV920. I would replace the rear wheel with one from a Seca650 or 750 to get an 18". I would replace the tank with one off an XS750 and cut down and recover the seat so it looked like a Corbin. Then put on some clip-ons and rear sets. I have been planning on doing one for a while and someday I am going to.
If it's the XV920H model, the one that doesn't look like a cruiser, then you should use that one. I think that one already had an 18" wheel.
And if it's the H model and you don't want to use it let me have it.