First of all, the MBB coffee chat is non-evaluative ... unless you completely mess up everything it's just a very nice occassion to get to know some McKinsey folks on a slightly more personal level. So you can really ask whatever you are interested in and might help to you make an even more informed decision.
The main idea is asking insighful questions which you cannot find out easily otherwise. For myself, I also appreciate candidates asking tough questions about the job and the firm - it shows also a realistic assessment of the situation and not focussing only one the "nice" things. Also, personal impressions or evaluations are kind of really interesting - like what turned out to be different at McKinsey for them compared to their expectations, etc.
Hope that helps - if so, please give it a thumbs-up with the upvote button!