People had been saying that the stock android is faster than touch wiz. But, now you can actually see the difference between the two. Here's a video by 'Android Authority' :

If you want to compare the performance, watch the video (by PhoneBuff) below:

Conclusion : Stock Android is a little bit faster with better benchmarks but there's just a little difference (neglect-able). The Disappointing thing in the stock android is that it has no cool features as the Touch wiz has. The camera app loses its charms in the stock version and, also there are no air gestures in the stock android. The updates come earlier on the Google Play edition but not earlier than the nexus devices.