There is a downside of being an open protocol: no certification. Zigbee vendors are free to mandate the use of their hub (and software, and spying) because there is no gatekeeper. And most certainly do. Philips attempted to do this somewhat recently, only backpedaling due to the bad PR this caught them.

Z-wave devices are guaranteed to work with any hub. That means that it, ironically, behaves in a more open manner. I consider that worth the difference in cost.