I'm a fan of Accuquilt and Creative Grids. I am currently working on a crazy quilt where I am using both brand's finished 6" square to get a more random look. After I had constructed about 30 squares of each which should have measured 6 1/2", I noticed the Accuquilt squares were measuring around 6 1/4" each. The creative grids were measuring 6 1/2". I was using the same scant 1/4" (Janome setting 8.7), Aurifil 50 wt. thread, and a Janome Red tip needle on both.
All of this is to say that if you use either, you need to create a test square to figure out what setting your needle needs to be on for the scant quarter inch in order to get the exact measurement for your squares.
The Accuquilt will be much faster. :-)