I realise this isn't practical for most settings, but I like the set-up at the Europeans, where you have a combination of set piece and own choice. This gives the adjudicators some common ground against which to assess standards, as well as allowing conductors/bands to choose something which allows them to play to their strengths. As a member of the audience, I would tend towards a set piece as I like to follow with a score, and would not fancy the practicality or the expense of having a dozen or more scores to cart around. There is certainly no better way to get inside a piece than listening to 20 or so bands giving their own interpretation - invariably, there will be something come out towards the end that you have not been aware of in any other performance, and which will make you look afresh at the score.