I have the Marilyn Doheny version and love it. I think there's an advantage to getting the 25" version, depending on what your making. The quilt I made with it required you to flip the ruler after each cut, so that's much easier with the 25" version.
Here's the quilt I'm referring to:
Just realized this is before it was finished. It has circles appliqued in the centers of each sprial.
[ATTACH=CONFIG]274529[/ATTACH]