I've owned and used the magnetic corner jigs for years. Not real impressed with them, they sorta work. They're OK for something like I'm building now - just a large bin/shelf thing to hold scrap metal. Not sure I'd use them for anything precise.
ETA: I also have one of the clamps in the OP. It's much better for precision work. Could have sworn I bought it at HF. It has been decades though, they must have quit carrying it.