Should I change the water or flour I feed my starter? I am using the same recipe provided by Chad Robertson's book 'Tartine Bread', only changing the water from Poland Spring to Nestle Pure Life Enriched with Minerals bottled water which is closer to a 7.0 pH.
Poland Spring is 7.3 and Nestle Pure Life is 7.2. It is highly unlikely that this is going to make any noticeable difference in your results. I'd say use whichever is convenient.
If your starter is working as you're feeding it now, there's certainly no reason that you should change anything. Sourdough cultures can thrive on many kinds of flour and in many kinds of water (with some obvious exceptions like highly chlorinated water).
Changing the mineral content of the water that you feed with could have some impact on the makeup of your culture, just as changing the flour could. It's very hard to predict what this impact will be as it would depend on which strains of yeast and bacteria your culture already contains. These changes would probably be pretty minimal.
In short, if you want to change it, go for it. If not, stick with what works. It probably won't drastically change anything either way.