What is the difference?
What Z-Wave and Zigbee share, beyond being wireless technologies, is that they’re designed to cover a range of simple home automation processes and share that information intelligently across a network of devices and controllers, so that, for example, if a set temperature is hit, an air conditioner might kick in while at the same time motorized blinds drop down to stop sunlight heating up a room.
From the home automation perspective, a fully installed Z-Wave or Zigbee system would probably be undetectable, because the endgame of home automation is identical between the two, even though the underlying technologies and even the philosophies differ quite markedly. Both are wireless, but Zigbee’s working within the 802.11 specifications, which means it’s closer to the Wi-Fi you’ve probably got working within your home, whereas Z-Wave is an RF (Radio Frequency) product.
From a technical standpoint, Zigbee can support more control nodes and devices than Z-Wave, with theoretical limits of 232 Z-Wave nodes to up to 65,000 for Zigbee. That being said, the practical limitations drag those numbers down quite markedly, and for just any home automation implementation unless you’re retrofitting a hotel you plan to live in, the number of available nodes should be sufficient.
To date, many Zigbee products were kept low-cost by only implementing networking features specific to their implementation, which is why some Zigbee branded gear won’t talk to other Zigbee branded equipment. Zigbee 3.0 should change all that, although you won’t find any Zigbee 3.0 equipment on store shelves just yet.
Conversely, while it’s proprietary, all Z-Wave products have to be licensed through Sigma Designs, and that licensing process means that all Z-Wave products talk to each other by design no matter the manufacturer badge on the outside.
Zigbee has the advantage of being standardized globally, whereas regulations relating to RF frequencies mean that Z-Wave products have to be built with United States frequency regulations in mind. What that means is that while you might see some cheap Z-Wave gear for sale online through an overseas reseller, unless it’s been built to work in United States, it’ll be useless for your purposes with an existing Z-Wave network in United States.
In most smart-home systems a central hub—like Vera, Homesheer, Wink and Smartthings—acts as a go-between between the various smart devices on your home network and the internet, letting you control everything over Wi-Fi when you’re at home, or from your smartphone when you’re out. One major advantage with this sort of system is that you can mix and match a hundreds of third-party devices (many of them low cost) because a hub supports a variety of wireless standards, including Bluetooth, Wifi, Z-Wave, and Zigbee, and is compatible with Nest, Alexa, and Google Home. HomeKit, in contrast, is almost completely decentralized: You can buy two or 10 or even dozens of devices and they will happily talk to each other over your Wi-Fi network with minimal setup and without requiring a hub as an intermediary.