As near as I know, wxCocoa is still in early stages of development. Also, I wouldn't count on it being magically superior to wxMac.
wxWidgets has some important strengths, but one of its weaknesses (due to its cross-platform nature) is that it can't take advantage of the special features of any platform's native toolkit. It takes Carbon... and make it work like wxWidgets. It takes Cocoa... and makes it work like wxWidgets. It takes GTK... and makes it work like wxWidgets. You get the idea.
Assuming wxCocoa were fully functional, it would still work a lot like wxMac, which is to say, like wxWidgets.