I personally tried them both as server administrator, and I found both cons and pros.
cPanel definitely have tons of functions, and is definitely optimized software. It supports lots of addons from bunch of software vendors, and it is very practical. I would recommend this if you do not mind paying, and your server can handle bigger load.
zPanel just refreshed their GUI recently, and I like its look. It does not support much of addons, and from my point of view, it is slightly depreciated from the point of view from other users (ex. XXX couldn't afford cPanel? zPanel sucks..). It is free, and I like where they are aiming for. Be aware though, there are less developers, smaller community than cPanel.
Speaking of my experience, I like DirectAdmin or Plesk better than this two options listed. Check them out; it's something worth doing.