In general though in a healthy agile implementation I believe it is not an either/or situation. It is not flow OR Iterations, but actually both. We need to commit to a set of work in the timebox, but within that there is priority, and given that a commitment is a guess sometimes we overcommit and sometimes we undercommit. In the latter case we bring things in to the iteration using a pull model (of course). In the former case we ensure we get the essential things done first. The timebox helps frame our work and gives us regular rhythm, which I find enormously helpful. It gives us regular breathing room.