Both Zigbee and Z-Wave have a wide range of products, making them equally viable. When choosing between technologies, the first factor to consider is application. In the case of Zigbee, if they are not all operating under the same protocol, the devices will not be able to talk to each other. This differs from Z-Wave, where all their products are compatible with each other. That being said, you can connect a lot more devices to Zigbee than you can Z-Wave. Providing that everything is compatible, and you have a multitude of devices to connect, Zigbee is usually the preferred choice.
Another factor to consider is speed and power. Z-Wave operates at a much higher frequency than Zigbee (908.42 MHz) and Zigbee is twice as fast, so is a better choice when needing a speedy response from devices. The only caveat is due to its faster speeds, Zigbee uses more energy.