The 20-1/2"...don't use it lots, but when I need it it's great for squaring up applique blocks.
I also use it for a faux-lightbox. I tape my applique layout sheet to the ruler (so it doesn't move), put an 'under-the-counter-fluorescent-light-stick' in an empty (shock!) Sterilite container (66 qt, I think) and then the ruler over the top...background fabric on top of that so I can place my applique shapes.
It's nice to be able to move the ruler around to get the best light for where I'm working.
Does that make sense?
I don't think I've met a square ruler that I didn't like...
Julie