It’s not a matter of one protocol versus the other, it’s just how you have laid out your local network, and in particular the “backbone“ you have of devices which are capable of repeating. Zwave repeats only for Z wave. Zigbee repeats only for Zigbee.
In both protocols, a Repeater is typically a device which is connected to Mains power. Battery powered devices do not typically repeat because it would eat up too much battery life. But there are a few exceptions.
Sengled has decided that none of their Zigbee devices will act as repeaters. The ring alarm system has a battery powered zwave repeater which is to keep the security sensors working even if the power has gone out.
Most smoke detectors, even if they are mains powered, do not act as repeaters because nobody wants a smoke alarm to be delayed because the device is busy repeating a message for a light switch somewhere.
So, as always, the first rule of home automation applies: “the model number matters.“
IT’S NOT THE PROTOCOL THAT MATTERS: IT’S THE BACKBONE
As long as you have laid down a good backbone, either protocol should be reliable. Z wave does have somewhat longer range for individual devices, but then it is limited to four “hops“ per message while the Zigbee profile that smartthings uses can have up to 15. So both protocols can cover the same total distance easily, although it might take more devices with Zigbee.
OTHER DIFFERENCES
Zigbee is a bit better than Z wave at handling high humidity situations, which is why it is slightly preferred for outdoor sensors and may be a little more reliable in a bathroom. But most people don’t notice a difference.
Zigbee has somewhat better power management, which is why you will notice that the inexpensive Zigbee sensors tend to be a bit smaller than Z wave sensors in the same price range. That’s because the Zigbee can use smaller batteries.
On The other hand, Wi-Fi can drown out zigbee, but does not usually interfere with zwave, so Z wave has historically been preferred for fixed point devices like light switches in DIY Projects where the homeowner may not have the tools to effectively identify and work around interference zones.
START WITH THE FAQ
Have you had a chance to read the FAQ on wireless range and repeaters? It should answer most of your questions.
Go to post 11 in that FAQ, read that, and then go up to the top of the thread and read the whole thing. I will link directly to post 11.
A Guide to Wireless Range & Repeaters - #11 by JDRoberts
But as long as you have enough repeaters for each protocol, you should not notice a difference in reliability.
So both are good protocols, it’s just a matter of your local layout. 