OK. I like a lot of different brands of rulers. I am back to using the sandpaper dots on the underside of my rulers to help me as my arthritis makes using rulers a tad difficult at times.
If I were starting out again I would stick with Omnigrid because of the line going all the way across and down the ruler. Start with the 6" x 24" and the 6" x 12". The have the lines for the different angles so you won't need specialty rulers probably at first. The I would add the Martingale/That Patchwork Place square up ruler for my 3rd ruler.
I prefer my Martelli mat. It has outlasted any other mat by far! It doesn't seem to dull the blades of my rotary cutters as fast either.
I only use the mat lines to line up the fabric. However, I do use my ruler to make sure the first cut - the trim cut - after folding the material properly, is straight. This is much more accurate for me.