Well, what's in "spring water"? That's the thing to know.
With RO or distilled water- you know where you're starting. You're starting with a blank slate, as all the numbers are effectively "0".
so you can dilute your tap water with good results, or use RO water only (and add a teaspoon of calcium chloride to it), with good results also.
Spring water is simply water that came out of the earth- it could be high in bicarbonates or it could be high in minerals. We'd have no way of really knowing what's in it. I know that in our area, a company bottles city tap water and calls it "Crystal Spring Water".