I have both, and I find them to be very useful. I, too, tend to use the 6 1/2" more, but when you need the 8 1/2", this is a really good item to have. In terms of sliding, the longer the ruler (not necessarily wider), the greater the chance of sliding. I just take my time, moving my hand from center to top as I cut. However, I've seen several videos in which the person cutting fabric with these rulers places a 3 lb. weight (weight bell) on the far end of the ruler and it doesn't slide at all! Creative Grid rulers are,by the way, wonderful! Alas, both my 6 1/2" and 8 1/2" X 24 rulers are Omnigrid and i just have too much money tied up to replace them with the Creative Grids ones. Best of luck in your decision making!