I have played both and I will say the Pioneer is way better than the S2.
Price wise it is a big difference though. The DDJ-SX is $1000 while the S2 is $500.
The feel of the DDJ is excellent. Quality built, great feel, laid out well and all the I/O you'd want or need.
The S2 is very nice as well, but only 2 channels of faders, no booth out and no dedicated filter control. That stands out as a problem for me. The S4 gives you those items missing except the booth out which is a big no no for me.
That said, do you like Traktor or Serato better? I like Traktor. I has Serato for years and it is pretty great, but I prefer Traktor now. The DDJSX you can map to Traktor, but it is not ready out of the box plus you need to buy the software.
I hope that helps some.