The SRT is newer with newer components, on board beat effects, platter display in the jog wheel, CDJ-2000NXS2 jog wheels (not the same platters as the SZ2).
The best Serato controller for me is the 1000SRT as it feels different from the SZ series when using it. I personally like the touch of the surface on the SRT and the UI as to where everything is place, and the fact that the price is a great value for what you are getting.