I don't see why there has to be an either/or.

When I go to a performance I expect the group or individual to have a set list. I've seen too many who want to 'go with the flow' waste time deciding what to do next. OTOH, I also hope they will be open to requests & be responding to their audience. Getting all confused when departing from the list is not good either.

If they tell stories & jokes they should know how to tell them. That may mean they've told them before. I like these a lot better than most of the tired old too-ning jokes.

A really good performer can have a plan, go with the audience & make it all feel spontaneous. If the performer is putting on an 'act' in terms of being something they are not, it probably will not work. If what they do comes from their basic personality & is 'real', the on & off script stuff can flow pretty seamlessly. Christine Lavin can do this. I also love Cheryl Wheeler's performances.