45mm is just a smaller blade to cut thru a few layers.. the 60mm will cut thru quite a few more layers than the 45mm. its all a matter of preference. Some think the 45 is easier to handler and others like the amount of fabric the 60mm will cut.. I have both and the 28mm for smaller cuttings that are more rounded.. Its a try it and see kinda deal..
I have one of each for sale if anyone is interested (listed in the classifieds..found them when I was cleaning my sewing room)
Quick Change Olfa 45mm & a 60mm