Hi, I have used a mixture of clicker and the whistle. The clicker for things such as initial stages of the present/walk to heel/sitstay etc and the whistle is a must for recalls/stops etc. I tend to click at the end of the exercise now so they know its complete, so on a recall I would use the whistle to bring them in, wait till they sit in front of me and then click. Mail me if that doesn't make sense.
Once they had each exercise I faded the clicker and tend to only use it when I want to tighten them up again in a training session, but the whistle I use all the time now, never go on a walk without it.