Funny you should ask--I have used a 45 but one day I had the opportunity to use a 60--loved it. Maybe that's because I am always in a hurry. But since I am getting a shape cut ruler than the 45 will be better for that.

I have a smaller one, the 45 mm, the 45 mm pinking shear blade one, and 2 60 mm ones. I love the 60 mm one from Olfa - it's their curved handle one. My girls bought me the Martelli for Christmas along with the blades (Very expensive). I use the 60 mm almost exclusively. Since I switched from the 45 mm size to the 60mm size, I just don't like the 45mm one. Now if I can't find the 60 mm, I do use the 45 mm and also if I'm cutting small pieces and sizes, I do use the 45mm one for better control. Haven't test driven the Martelli yet but am thinking about playing with it today after I get going on some laundry.

I use them both, the 60mm for cutting through more layers, the 45mm for regular/normal cutting.
There is a cost difference in the blades, I can usually pick up the 45mm much cheaper on sale than the 60mm :D:D:D

Always used thr 45 mm, then bought a 60 mm with a special handle for AR. Love it, don't even look at the 45 mm any more.

I have all the sizes available for the rotary cutter. I find that I rarely use the 60mm. My favorite is the 45mm in the ergonomic cutter. It gives me the most control and is most comfortable to use. I use the 28mm to cut out garment patterns--especially helpful with slippery, silky fabrics. I have only used the 18mm one once in a class for miniature quilts. I think I could have accomplished the same thing with the 28 mm. Hope this helps.

I recently misplaced my rotary cutter and treated myself to a new 45MM Gingher. Wow! So smooth. Worth the extra $$$.

I have both and use which ever one is the closest at the time. Both are good in my opinion.

I use the 45 mm versus the 60 because it gives me better control. Although I never cut more then 4 layers to get correct accurancy, I feel a 60mm is not needed.
CruisingStef or Stephanie

Unless you are going to purchase a new cutter, you use the size that fits. If you do lots of 'fussy cutting', or intricate cutting, a small blade might be easier. 'Fussy cutting' can be much easier with a small blade. I have never used a 60mm, but I can see where one would be more useful. (Cutting backing fabric, etc.)

I have to join the ranks of the 60mm. I used the 45 for years and then got a 60 for the fleece cutting I was doing, it's so thick, and now I find myself grabbing it first most of the time