Really just depends on how you have your network laid out. The zwave one will repeat zwave, the zigbee one will repeat zigbee.
So draw a simple dot sketch of your two existing networks, Z wave devices on one page and zigbee devices on another. Circle the ones which are repeaters (anything mains powered except a smoke sensor). Battery operated devices are not repeaters. Put a plus sign on any Z wave device which is Z wave plus.
Don’t include any devices which are attached to a Hue bridge – – they form their own mini network and won’t affect plug selection. Do include the smart things hub on each page – – the hub itself is a repeater for both protocols. (I put the hub in a square so I can remember which one it is. Some people market with the star instead of a dot. Whatever works for you. )
Try and draw it approximately to scale, typically using a half centimeter on the paper to represent a meter of space. ( you can use a smaller distance as long as you can be pretty consistent as you go. If you have graph paper, you can use each square to represent 1 m.)
You don’t have to be perfect, but you want to have some sense of how far apart your existing devices are from others of the same protocol.
Typically, you want a zigbee repeater about every 12 m. Zwave plus devices have a considerably longer range. You can use about 20 m for those.
It does vary depending on your local architecture – – if you have brick or cement walls, you won’t be able to get signal through the very easily. But you probably already know that about your own house.
Anyway, after you’ve laid down all the dots, you should have some idea of where the backbone of your mesh is weak. That is, where could you use another repeater? That should help inform your selection of the plug.