I have both but mostly use the 45. This is because Martelli blades suck (for me) and it's harder to find 60mm other brand blades. I have lots of 45mm blades so I use that one instead. I only use the 60 if I have to use a thicker ruler (like my piping ruler thing).
The only reason I have both is because I ordered the 45 and they sent me the 60 in error. I called to have it replaced and they talked me into keeping the 60mm at half price (big regret). I like this style of cutter but the blades are terrible.