I have all sizes. I prefer the 60 in my Martilli but it is dull. I am really ticked at the salesman who sold me it at a quilt show. He kept saying the blades would stay sharp while trying to sell me extra ones. I went to a smaller show this past weekend and there was a woman dealer there. She said the blades would dull and was not surprised that her male counterparts had said otherwise.
I have an autoimmune disease that effects my muscle strength and I tire very easily. The larger blade and ergonmic handle helps me be able to cut fabric longer. I still have a problem with keeping it straight on longer cuts.