I've got an SL4 Pro and rode a demo S-Works Venge the shop had for a few days last year.

The Venge obviously looks better and is supposed to be more aero. Neither one was more uncomfortable than the other and they both handled about the same IMO. Riding each one felt different, but they are different bikes so it was expected, especially since the Venge was a 56 and the SL4 is a 58.