Typically the range is 40 feet for zigbee, a little bit longer for classic zwave, and around 60 feet for zwave plus inside a US style house that has a lot of wood and is not airtight. The range will be shorter for brick, cement, or Adobe. Water pipes and certain kinds of insulation inside walls can also be a problem.
So in a cement house you might only get a range of about 15 feet per device.
Mesh networks work by having repeaters pass along messages, like a relay runner passes a baton. Both zigbee and zwave in a SmartThings installation are mesh.
Zwave allows for a maximum of four “hops” Per message, while zigbee allows for up to 30 (15 into the hub and 15 out again). So although the range per hop is longer for Z wave, you might actually get more coverage from zigbee, especially if you have to bounce signal around local obstacles like brick walls.
The following graphic shows the reduction in signal for different types of materials.
In an ideal situation you would have two repeaters for each protocol every 40 feet or so. ( again, in a house with cement walls and floors, as is fairly common in Asia, it might need to be One near every doorway.) Typically light switches. But that’s a lot of devices, and not everybody wants to do that.
If you want to save money on repeaters, use zwave plus for your battery powered devices and put a Z wave plus light switch where you want a smart switch and use the iris pocket sockets as needed to cover dead zones. That would probably be your least expensive network backbone.
But most people fall somewhere in between. They become enamored of the cheap zigbee sensors, then they need zigbee repeaters but they don’t want to use zigbee light switches, so they just fill-in with pocket sockets. 