OK, went to Lowes and found both of these on the shelf. So what's the
deal?

I understand that the "substitute" has no phosphates, and there are
communities that properly discourage phosphate pollution. My community
isn't one of these (unfortunately), but then why are both on the
shelf? Is environmental protection all there is to this?

I'm not using a lot, and I frankly would just like the chemical that
works best. Which does? The price is the same. I'd be happy to use the
"substitute" if it doesn't work much less efficiently. If the "real
thing" works better, then I'd just get it, and use it carefully.