I love my Shape Cut. The great thing is that once you've cut strips, you can turn the Shape Cut and cut squares and rectangles without moving your fabric. Cuts your time in half (no pun intended <LOL>).
My basic rulers are a 6-1/2", 9-1/2" and 12-1/2" square and my 5 x 24" ruler. At the time I was first buying tools, the JoAnn's I was at didn't have any 6x24, so I got the next best thing.
As a beginner, I would never have purchased the Shape Cut (I wouldn't have wanted to invest that much in a hobby I did't know if I would continue doing), but I definitely would get the 6-1/2" square. If you know you're going to continue quilting, I'd recommend investing in the Shape Cut