I would say go to the cheapest option, as well. Right now that seems to be UNC, and luckily for you, UNC is a top public health program.
Loans are no joke, especially the unsubsidized, and if you get no aid from Yale, then I think it's a no-brainer. Especially if you decide to pursue a PhD afterwards, you will be making close to nothing on the PhD stipend while having to pay back those enormous loans. I personally don't think it's reasonable to go into incredible debt for an MPH especially due to the career salaries afterwards. Just my two cents!
I'm in the same boat cause I think I'll want to get a PhD after and the idea of loans frightens me, especially upwards of 20-30k+