Seeing as tap water normally has fluoride added too I would stick with the tap. That said this Australian fact sheet is good, https://www.cyh.com/HealthTopics/HealthTopicDetails.aspx?p=114&np=302&id=2709
I've been looking into this recently after a chat with the HV about fluoride toothpaste for LO, and I can't find anywhere that tells me any more than about 20% of our UK water has added fluoride. It's only the midlands and a couple of isolated areas, the majority of the UK is added-fluoride free as far as I can see
Feel free to correct me if I'm wrong though