If I’m not mistaken, ZigBee devices tend to be more readily available in European markets at a reasonable cost. The downside is that ZigBee runs in the same frequency band as WiFi (although there are some gaps between the actual frequencies used to mitigate interference). The upshot to Z-Wave is that it runs around 915 Mhz so no conflict with WiFi (most consumer RF devices) and you tend to get better obstacle penetration (walls). The problem here is that Z-Wave operates in different bands for different markets. A US device would not work (out of the box at least) with a European controller.
I run a mix of both networks and can’t say I have any real complaints. The key to either will be to make sure you have enough repeating devices (both protocols create a mesh and will use each other to get back to the controller). General rule of thumb is that any mains powered devices will act as repeaters. No matter what you land on for switches, I’d highly recommend they support “smart bulb” mode. These switches allow you to decouple the paddle/toggle from the actual power relay (meaning the paddle will no longer turn power on/off). This will save you plenty of headaches down the line if you switch to smart bulbs anywhere that are reliant on always having power.
One thing to keep in mind with WiFi devices is saturation of your WiFi RF. My approach is all IoT devices go onto the 2.4 Ghz band (most only support this anyhow) and any people devices (e.g. phones, laptops, etc) go onto the 5 Ghz band. Easiest way is to have separate SSIDs for each band versus trying to rely on band steering. I also have the IoT network set to its own VLAN.
Here’s a good primer to help with segmenting off the IoT devices:
Here’s a recent thread with some good tips to have in mind through your journey: