I have both, and each is very good. However, I find that I use the 12 x 12 more. It is a handier size and is at the front of my "ruler rack." I use mine to cut everything. I don't think I could quilt without this ruler. The 12" x 18" would cut more strips, but the 12" x 12" does that too...just fewer at a time, obviously.
Dina