The main difference will be the parts per million (PPM) of total disovled solids (TDS) that are in the waters. TDS consists of all kinds of different stuff, some can be beneficial but mostly not. The tap water around here runs anywhere between 350PPM to 500PPM. I just so happened to have an unopened bottle of aquafina right here and it just tested at 3PPM. It will also have less chlorine and such in it. The water from those "fill-up" stations on the street test pretty low also.