I have always used45mm -shopping at walmart one day and they had 60mm blades 50% off so I bought 3 the bought the cutter -I prefer the 60mm now.

I have used the 60 mm one for cutting thru batting, trimming the sides of a quilt after quilting, and even cutting heavier fabrics. But most of the time I use the 45 mm one. As someone pointed out, blades are cheaper for the 45 than the 60.

I usually use whichever one I find first.

Starch and press your fabric before cutting improves cutting accuracy. Helps prevent bias from stretching and also makes cutting for mini quilt pieces and applique pieces more accurate and easier to work with.

Good information. I use 60 mm and to get straight of grain I fold fabric then line up the bottom folded edge on the bottom of the cutting mat, then using the lines on the cutting mat line up a ruler to make your first cut on a straight line sliding the ruler towards the cutting edge then cut across the fold to the open edge. You might lose an inch of fabtic to get a straight line, If i need 2 inch pieces I cut 6 inches then slide the ruler to the 4 then 2 this keeps the fabric together and you get more cuts without lifting the ruler or fabric. Hope this makes sense. Use the mat and your ruler as a cutting guide.

thanks guys for all the tips and pointers. will see which one works for me. :)

I have both but like the 60mm best for control. It won't eliminate dips. There ways to do that - Harriet says to starch first - I have found that pressing first helps - good luck

When making straight cuts you also have to remember the width of your ruler matters too. I fold my fabric twice so I have four layers. I line up the top of the ruler at 1/2" and the bottom must line up on any line at the bottom. If both don't line up then the fabric isn't folded straight. Also if the ruler is only 6" wide that is all you've squared up. Ex: you want to cut 8 2" strips. The first three cuts are square if you started correctly. Then move the ruler so the whole ruler is on the fabric and trim the cut edge after you line up the ruler at top and bottom. Then you can make three more strips. What I'm trying to say (I sure I've made it as clear as mud) is the wider the ruler the more cuts you can make without double checking the cut edge. Most of the time I use my 12 1/2" square ruler so I can make more cuts before squaring my cutting edge another time. Hope I've helped and not made you say "what is she talking about?" Later today I'll see if I can find the tutorial. I think it's the one Alex Anderson did.
Oh, and to answer you're question I use my 60mm most. Have all sizes but like it the best.

Use the 45 and the 18 mm for intricate cutting. I always pre wash the fabric, and press before cutting, thus it is clean, I know if it will run, it is pre shrunk, and not bolt wrinkles. Takes more time, but I feel it is worth it in the final product, since all fabrics do not shrink at the same rate, etc.

Those dips are not due to your cutter, it is the fabric. When you carefully line up the fabric for that first cut you make sure it is lined up at a true 90 degree angle. However as you make subsequent cuts the angle gets smudged. You have to realign the fabric every two or three cuts to avoid the wonky strip. The reason the shorter cuts are not wonky is there is less risk of getting out of alignment. I hope I am being clear, and not confusing you. I line up the bottom fold of the fabric and the side I am going to cut on the square on the cutting mat. If you get a wonky strip, check the alignment and you will see you are cutting a piece that is not on the square. Re trim and go forward.