I have the Havels Quilt Ruler Cutter. I like how safe it is, nice crisp clean cuts and easy to store. I too have Fibromyalgia and difficulty with my wrists. My issue with the Havels cutter is that I don't have enough strength to push down on the cutter part all the way to the end. Maybe it's the lack of strength in my hands but pressing down on the cutter is more difficult for me than I expected it to be. I really like it very much but it hasn't resolved my issue with cutting.
I have the Quilter's SlideLock rulers and have no issues with them at all. They are so easily stored, fabric doesn't slip and affordable. Good luck with your choice!