The clamp set (comprising two clamps, one with a handle) is a much more mechanically complex solution than the clips. I happen to think that it is also less elegant. Nonetheless, I was intrigued enough by the idea that I bought myself a set. It even came in a box, labelled ""Portable Oil Painting Clamp":
Brass, I think. It's advisable to start with the bottom clamp (the one with no handle), and to put one canvas in at a time. I found that it is advisable to loosen the screw all the way and to be gentle when tightening over the edge of the canvas. Once both canvases are held by the bottom clamp - and the top edges are doing their utmost to close up - then you can turn it round, and put the top clamp on.
I advise lots of practice.
I kind of like the handle, even though it's unlikely to help me transport it around my usual painting haunts (where both hands may be required and the best place for a pair of canvases is on the back of a rucksack). But it looks the part: