Hi Sean,
I bought an SX2 last year and haven’t looked back, but I almost caved in and went for the S4 as it’s a great bit of kit too. If you sure are that:
1. You want to be solely in the Traktor ecosystem for the foreseeable future
2. You want something that’s very portable for taking to gigs
…then get the S4. The software/hardware integration doesn’t get any better for Traktor after all.
Get the SX2 if:
1. You don’t need something smaller
2. You want the flexibility of being able to use Serato, Pionner Rekordbox DJ AND Traktor all in the same controller
I have only used the S4 in a music store and had a good play. If I could own both then I would.
Good luck with your final choice but do think carefully which direction you go in.