I've always liked the postwar ZW and consider it the gold standard for any transformer. The output is absolute PURE sine wave and all of the other features are not hard to add. That being said, the Z4000 has all of the features already added, and generates a sine wave that is about 97.34% pure. It has more than ample power to run two tracks.
The ZW is more affordable especially right now as the price tag on all of MTH's power and control systems seem to be sky rocketing.
Both are great, Which ever ever way you go you won't be unhappy.