Found this link:
Lots of good info, but here’s the summary if you don’t want to read the whole story:
ZigBee and Z-Wave target the same general applications. Of the two, ZigBee is by far the more versatile since it can be configured for virtually any short-range wireless task. Profiles are readily available to minimize development time for common applications. On the other hand the protocol is far more complex, resulting in longer development times. Z-Wave uses a far simpler protocol, so development can be faster and simpler.
Z-Wave chips are available from only one source, Sigma Designs. They sell only to OEMs, ODM, and other major clients. More than 500 consumer home control products are available in stores like Home Depot and Lowes, but many don’t state that Z-Wave is used.
ZigBee chips are available from Ember, Freescale, GreenPeak, Microchip Technology, and Texas Instruments. Complete, ready to use ZigBee modules are also available from multiple sources like Atmel, CEL, Digi, Jennic, Lemos, and RFM.
For a given power level of 0 dBm, Z-Wave’s range is greater than ZigBee simply because the lower operating frequency supports it with pure physics (Friis formula). That also translates into a more reliable connection in some applications.
ZigBee uses the widely populated 2.4-GHz ISM band, which it must share with Wi-Fi, Bluetooth, and other radios that can produce interference. Most ZigBee devices have co-existence features that help mitigate interference, yet the potential is greater in the 2.4-GHz band than the 908.42-MHz channel of Z-Wave.
It would appear that Z-wave has a stronger signal, and shorter development time, but is also a closed system and only available to large scale OEMs, etc. Zigbee is weaker signal and might have issues with interfering with BT and WiFi, but is far more versatile and readily available to anyone.
It would seem that Z-wave is the stronger, more reliable choice. But the fact that it’s closed and you probably have to pay a royalty to the Z-wave people means that Zigbee likely isn’t going away anytime soon. Garage-workshop developers will likely gravitate towards the openness of Zigbee.