Much thanks for the helpful feedback.
So, yes, the subfields are very very different in both schools even though it's the same PhD. To give a close analogy, let's say I'm getting a PhD in Psychology. In the future I want to do work on youth, and like you said, it really doesn't matter what research I do, anything is fine as long it is related to youth. School A has no faculty members doing research on youth, but plenty on older adults. I find this interesting, but I will never see myself working on older adult populations in the future. The opposite for School B, it has very intriguing research on children and I was so close to accepting until I got the offer from School A.
At this point, I don't know my next steps after PhD, but it'll likely be PostDoc, but I want to work on youth research. I'm in such a dilemma..