I have always used Omnigrid rulers as well. Someone already told you to stick with one brand so they measure the same. Although I have always used the green Olfa mats, this time I wanted a different color-the green gets boring after so many years! I bought a blue Dritz and love it. I can't tell a bit of difference between the two brands.
Even though you may not want to ever have to buy these things again, they do wear out and will eventually need replacing.