After reading on the board here, how experienced members had a moment of distraction with a rotary cutter and were emergency room bound, Painful recovery times....I wince just thinking about it.
I was convinced the Slide Lock was a necessary item for my sewing room. For me there was the learning curve of using two rulers one with lines to measure and then placing the Slide Lock next to the ruler and then removing the lined ruler. I cannot understand placing the Slide Lock on top of another ruler as other's have stated, I would think you have defeated the purpose of the Slide Lock locking onto the fabric to keep the ruler from sliding?
I first bought the smaller one because the demonstrator suggested it was her 'go to' size. I used it and loved it eventually purchasing the larger one which is probably my 'go to' size preference.