I use the 60mm cutter for thick fabrics - fleece, flannels, denim, corduroy- I use the 45 for regular cottons, 2-4 layers. If I’m cutting more layers I grab my 60. So, for basic cutting regular quilting cottons the 45 is all you need, if you make flannel quilts or back quilts with fleece - Minky, etc the 60 might be needed. ( I have cut plenty of flannels and fleece with my 45, it’s just easier with a larger blade )