Zigbee is better...
- wakeup speed, which translates to faster motion detection. Not more reliable, since they all work from the same PIR toolkit. With PIR technology being 20yrs old, there's not a lot of unintentional variability.
- Setup or Pairing is simpler because there's half the per device assignment of addressing. Zigbee device address is hard coded from the factory. Like ZWave, your specific home network address is added during pairing. All of this translates into re-pairing a Zigbee device if it falls from the mesh is more pleasant. The device just pairs back the same as it was.. the hub detects the device address and drops it right back into the config that existed before.
Zigbee is worse...
- Interference potential. Two types of interference are usually bundled together: physical and RF. Zigbee, runs on 2.4ghz. Perhaps you have one or more of these devices that also use the 2.4ghz range: WiFi, Bluetooth, wireless Phone, car alarm, microwave oven? On the physical side, everything attenuates RF signals but 2.4ghz is affected more than lower frequencies. ZWave at 800-900 mhz is affected less by walls, etc. and Lutron's ClearConnect at 433mhz is affected the least (of the RF we generally use in Home Automation.) The standard solution is to add more Zigbee AC powered devices.
- Product choice. Zigbee has a much reduced range of product types than ZWave. Need an in-wall Zigbee outlet to be a repeater for your mesh? Almost unavailable. The choice is plug-ins that need red labels so that the people in the house don't unplug them.
That's my list of factoids.. but not even close to exhaustive
If you have a bad mesh, you will be much happier with Zigbee because you'll be re-pairing often and that is so much more satisfying with Zigbee. Building an adequate ZWave mesh is a little easier if you're looking to swap wall switches, dimmers and outlets. They are AC powered and you can't unplug them,
I have an adequate ZWave mesh built up over years of adding devices every few weeks or months. I kept away from Zigbee until I just couldn't wait any longer for a canopy Fan/Light and had to get 4 of the Hampton Bay Zigbee Fan Controller. Sticking any radio inside a metal bowl (canopy) is a horrible idea and the Community recommends 1 or more repeaters per fan, It's not like the Hampton Bay Zigbee Fan Controller are cheap, but adding repeaters makes them pretty expensive.
I have one Zigbee network in my home, and building out the mesh for the Hampton Bay Zigbee Fan Controllers has given me a good mesh, that allows me to add in Zigbee battery devices, like motion sensors.