I have four: 60, 45, 28, 18. The 60 I use for batting and lots o' layers. The 45 I use most b/c it has the ergo handle, cuts most everything just fine, and the blades are cheaper than the 60s. The 28 I use for small cuts, like those Marti Michell templates, and geting just one square out of a large-ish scrap. The 18 I bought b/c the LQS promoted it for cutting the Marti Michell templates but it really only does 1-2 layers well and dulls quickly so I hardly ever use it.

Same here, i also use the 60 for squaring off after the quilting is done, all those layers...

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

I have found uses for all four: 60mm, 45mm, 28mm and 18mm.

I have all three sizes. I only use the smallest one on curves or tight cuts. I do use the 45mm the most, but I use the 60mm for thickest cuts. I use my 45 to cut out clothing patterns too. For me it's much faster. I've been using them since they became available. I've been sewing for over 50 years. Jan

I have several sizes but use my 28mm the most. It seems the most comfortable in my hand.

I have both and use both but don't really think it makes any diffrence which one you use. They do cut the same.

I have a 60, 45,28 and i Think a 15. Use them all for different projects. The 60 I just got so haven't used it much yet - just for large blocks - faster. The smaller ones I have at my machines for cutting the dog ears or for very small blocks.

I have both & use both.

The small one is (I believe) a 28mm and I don't know about the 15. I feel the 28 is great for cutting small pieces and pieces with small curves or circles