I second the idea of going to the store and trying the rulers on a mat and on the fabric. An inch is an inch but on some rulers you cut to the outside of the line to get the inch and others you dont.
Adopt a cutting system (matt or ruler only) and a ruler company would be a good start. Get a 24 inch ruler (I use my 8 1/2 by 24 but that's me) and a square. Then next one I use alot is a 2 1/2 by 18ish.
I own several, whole drawer full, but I use a couple constantly and the others rarely. But when I do want them, they are there. Just pulled one out of the drawer last night. Practically had to dust it off but was glad I had it.