SS's are better looking and better finished, but GP100 have a more consistent, smoother DA trigger pull. SA pull is very good. The SS's seem to have a heavy spot in the middle of the DA pull.
I shoot my GP100 in DA better than any other revolver I've owned.
The quality is very good on my new-last-year GP100. Very smooth chambers, throats and bore, and the Hogue grip soaks up the recoil very well.