You might ask yourself that if you had your SIL's skills what would work best for you? I'm a master carpenter, antiques restorer and home handyman. I have a classic Workmate plus one I converted into steel tops for welding. I weld quite a bit.
Neither one comes out more than a couple times a year. I do use anything that I can place flat on anything all the time. What do I mean?
Well, plywood on trash cans gets used a lot. I have 2 old folding card tables I would hate to be w/o. I have folding, adjustable height sawhorses that work very well, but are a PITA to set up quickly like the trash can deal. Then I have 4 T stands that make a good setup even if different heights are needed. Those are hard to store out of the way.
So, it's a mixed bag. Find out what he does do and what is needed. Personally, if I had to reduce to only one item, it would be a folding table with some area to it. Something a Workmate doesn't have.
You could, however, make a sheet with a stringer on the bottom that would clamp into the WM. Now you have a table and some jaws if you need them. I guess I never thought of that since I have several options.