A frame jig's purpose is to hold the frame components in a fixed location during the welding/straightening/bending processes. Given that, the frame jig needs to provide for a reference for your measurements.....usually a flat surface. The frame jig must resist the forces exerted by your construction.... for instance, if you need to "tweak" the frame by slightly bending it, you would clamp against the frame jig and pull (or push) the frame into place. If you had a wimpy frame jig, the jig would bend and your reference would be off.

Therefore, the answer regarding material is: STOUT
You can use I beams, rectangular tube, square tube or whatever is available since the price of steel went up. You cant use stuff like 2 x 3 rectangular....it isnt strong enough. C channel will twist too easily......STOUT is best.. maybe you can get a couple of bridge I beams!!

Some people use a welded tube frame and put a thick plate on top of it.. something like 1/2" thick. That way, you can attach stuff by drilling holes or welding tabs on the table top.