I definitely use the 6x24 the most because I usually cut strips. Rather than folding the center fold back up to the selvage to cut 4 layers, I fold end to end of the fabric piece. I am still cutting 4 layers at a time, but it is easier to line up the folds and the ruler to get straight strips. And it is actually cutting 2 strips at a time.
After 30 years of quilting I finally got a 12x12. It does make squaring up easier.