I also have both. I can't cut straight. The ruler doesn't slip, the fabric does. I wanted to cut a 1" strip from a 1 1/2 strip and wound up having to tape the strip down to cut it. Tired of the nonsense. I've tried just about everything else, and still can't get straight cuts. Looked at the June Tailor rulers and decided they were not for me. Started using an old Sizzix, and things went so much better.
I decided I wasn't cutting 49 LOF 1 1/2" strips for Jaybird's Night Sky!