This is a good question. Furthermore, it is great that you have two questionnaires. In light of this, you can do a lot of creative and helpful things. Let me give you some suggestions.
First, you can ask customers a number of questions on many different topics. For example, you can ask: how do they view the customer service, the speed and the helpfulness of the staff, the kindness and competence of the staff, and the like. You can also have the customers rate things, so that you can get a more nuanced sense.
Second, it would be interesting to also interview the workers to get their perspective. The questions can be directed not only towards customers and how to serve them better, but also how the employees can make the company better. This last point is important, because if the work place is better, most likely will customer service.