I would like to build a good backbone Zigbee mesh with the Ikea's devices.
Now, which is the stronger and/or more reliable between:
Tradfri's Signal repeater
Tradfri's control Outlet
Tradfri's Wireless control outlet
I've used those.
It appears to me (totally unsubstantiated) that the signal repeater has a stronger signal than the control outlet. However, as someone called it a "wall wart", it's not for everyone.
The real goal is to build multiple pathways between your devices and the HE hub so stronger is not necessarily better. I prefer to use, wherever possible, a wired device that actually contributes usefulness as opposed to a plug in device whose sole purpose is to repeat signals. Plug in devices generally go into outlets that are close to the floor and the radio signals are more easily blocked or absorbed by furniture, appliances etc. Plug in devices are also more prone to accidental or inadvertent removal, causing mesh disruption. Switches, are usually mid wall and are better placed to send and receive signals and in my opinion, make them a much better choice as a functional device that also repeats.
Unless you have metal junction boxes and switch plates. Fortunately for me, My house has the plastic boxes and I've used mostly nylon switch plates, because I would much rather have a function attached to the expense, not an eyesore.
Just an FYI. The Tradfri Control Outlet and Tradfri Wireless Control Outlet are exactly the same; it is just that one of them (I forget which) is sold with a button to control the outlet.
I have five Tradfri outlets and three Tradfri repeaters to support ~25 battery powered zigbee devices (Xiaomi sensors). They both seem to function equivalently. I've see no difference in the number of zigbee end-devices routed through either.
P.S. Welcome to Hubitat! I think it is great that you are taking care to build a strong zigbee mesh - it will definitely make your Hubitat migration easier/better.
I remember using a wooden dowel and mallet to remove these when I redid the wiring in the upstairs of my house. The downstairs was stripped to the studs after Katrina - so that was a lot easier to rewire.
FYI, metal boxes have minimal to no impact on Lutron or Z-Wave transmission. Otherwise they would not be usable in Chicago metro area, which is a conduit, metal box code area. Not sure about Zigbee as there are not many in-wall devices and I don't have any.