While my 6"x24" and I are old, old friends, I was surprised to find that my 3"x12" is forever getting used for one thing or another. It is so very handy, travels well, is easy to read and great for trimming up those smaller pieces or cutting apart those assembly-lined pieces sewed to a strip.
Those would be my two strong recommendations: First the 6x24, then the 3x12.