A protocol (or any standard for that matter) is only as good as its acceptance and support throughout the industry. Right now Z-Wave is clearly ahead of any other protocol in terms of device availability and interoperability.
From what I can tell, Thread is just a 6LowPan (TCPv6 over 802.15.4). It’s a fine protocol, but where’re the devices? There’s virtually none. Also, claims that Zigbee devices can be ‘updated’ to run Thread are grossly exaggerated. It’s true that the same hardware can technically run both protocols, but most of them are not field-updradable.
Here’s an old discussion on the subject: