I have the 28mm,45mm and 60mm. I bought the 60mm cutter when I took a class for the wonky 9 patch(stack and slash method). I needed to cut 9 layers at a time.
I use the 45mm cutter for smaller cutting jobs, such as when I'm cutting half square triangles after sewing from squares.
I use the 28mm when I'm cutting circles or drunkard's path
patches.
Another Sharon