I own a W4S ST-1000 MK2 and also had a ST-500. I owned the PS Audio M700s briefly.
They were in different systems, so I haven't compared them using the same speakers. The M700s seem to be more dynamic while the W4S are a little smoother on top. Over the long term the W4S might be a little easier to listen to.
Having said that, I didn't really notice the little bit of an edge that the M700s have until I took them out of my system and put my Herron Audio M1's back in. I didn't sell the M700s because I didn't like them, I bought them as backup while I had the M1's in the shop. I was actually surprised at how good they were at their price and was tempted to keep them.
I don't think you would go wrong either way.