In any case, the difference between a cake with arsenic and a cake without arsenic is going to be significantly less than 1%, but I'm going to make my decision based on that less-than-one-percent.
For health reasons. They're very rarely criticized for their environmental impact, when they're impact is almost certainly greater than bottled water (since they're not just water and a plastic bottle, but water, a plastic bottle, plus sweeteners and additives).
Some schools have banned bottled waters but allowed bottled soft drinks.
Tap vs. bottled water is more perplexing because it's basically the same thing in both cases. It's more like going out of your way to go to a paying park instead of going to the one next door even though they're basically the same.
I hardly drink any soda by the way, so it's not like I'm arguing that they have great value.
I don't have a water tap in either place, so it's not an alternative.