If its going to be used for assembly and rework, then IMO, a camera based unit like the tagarno is not great. Although I know some people here are OK with that. Depends on preference.
As jayx said, first one is overpriced for what you get, no zoom.
Mantis can be good, some of us here don't like them. As mentioned, working distance is not great, and FOV is not good. Get a demo unit or go try it somewhere if you can.
"We have an old manual binocular microscope on an arm but it's awful to work through and the optics are distinctly average. "
Unless its a cheap piece of garbage, damaged, or dirty, the optics won't make much difference for electronics work.
Why specifically is it awful to work through? Maybe you need better lighting? Ergonomics?