Northwestern gives scholarships to many students, so $ could make this decision very easy in a few months.
Higher CoA in Streeterville it's just better than HP in regards to living situation, things to do around there, proximity to 'fun' and obviously the Lake/atmosphere.
Prestige is probably a nonfactor since you don't want to do anything competitive—even then, it's pretty much the same.
Both schools will give ample opportunity to work with underserved and/or Latino populations (both have a free clinic dedicated to latino/hispanic patients iirc) and there are plenty of communities in Chicago that I'm sure both of these med schools have ECs/programs that give you exposure.
UChicago and Northwestern, while sharing a similar mission, do seem to put different things at the forefront. UChicago is probably the #1 school in the nation for social justice, health equity, etc. while Feinberg seems to be a bit more research/academic intensive (obviously both schools are top tier in both of these aspects, but this was just the vibe I got).
Honestly a lot of options here for you and since you and your partner are 20 min away from each other, it's not like you need to attend the same school to be living together/seeing each other often. Maybe even could just find a happy medium and split a 1 bedroom closer to Feinberg if schedules permit.
Also keep in mind UChicago still has a traditional 2-year preclinical while Northwestern has a more 'new' curriculum.