Firstly, remember that I am also using a fountain, so the water is always flowing, which most cats seem to like.
That said, I went to distilled water because of Feline Lower Urinary Tract Disease (FLUTD) is also known as Feline Urologic Syndrome (FUS). Since the urinary tract was getting crystals, many vets will suggest distilled water so there is less impurities in the water that can turn into crystals. Beyond that, I think it is better to not have the crap found in tap water (which can range greatly depending on where you live, the source, etc) BUT no tap water is as pure as distilled water. Now I continue to use only distilled, but then again we have well water that is VERY hard (265 grains!) and we won't even drink it. So if you decide to get a fountain, try tap distilled water (not to be confused with spring water or purified water which have more impurities than distilled).