@cuyler You’ve asked a good question. I’ve used both when I got my autoimmune disease. Each has its pros and cons. The walker ( with aluminum frame) can easily be folder up to fit in the car or placed out of the way. You need to have a “basket” on the front if you want to carry anything. The rollator is much bulkier and doesn’t fit in cars easily. One just needs to learn how to use it safely. My mother lived in a retirement facility and , I think, almost everyone there used a rollator. They decorated them to go with the seasons or holidays.
I’m also a nurse, so my ‘nurse self’ thinks that your mother is safer with a walker. If she’s not given a choice, just watch her closely to make sure she is safe.