You mention that cost is not an issue, and yet list cost of living as a con for BU and a pro for UF. If you're OOS for UF, then I imagine CoA will be very similar for both schools.
I'm making a similar decision with UF vs. an expensive OOS private school, and I will probably end up attending UF. I'm an older non-trad than you, but I am pretty confident that I would be happy in Gainesville for 4 years. It is a college town, but remember there is a large grad student and faculty population so it's not just 18-22 year-olds. Nothing can match living in a large, diverse city, but you may find you like life in a small town at least for a short time. The weather is wonderful in Gainesville from October/November to April/May, so you'll probably find yourself happier during the winter months. There are lots of outdoor activities to do, and the student body seems close-knit. Being able to walk or ride your bike to campus year-round is a reality. It will be a very different life than living Boston, but that's not necessarily a bad thing.
If you look at residency director surveys, only 50% of residency directors list "being a graduate of a highly regarded US medical school" as being a factor at all in selecting for interview, and only 38% list the same as being a factor in ranking the applicant. I don't know if there is a big enough difference in the ranking between BU and UF for it to be significant during residency apps (I think this more comes into play when you're talking about a UF grad vs. a grad from a T20). There are many other more important factors, like board scores, letters of rec, class ranking, audition rotation performance, interview performance, personal statement, prior familiarity with the applicant, and demonstrated commitment to the specialty (
http://nrmp.org/wp-content/uploads/2018/07/NRMP-2018-Program-Director-Survey-for-WWW.pdf).
If you are committed to matching a residency in Boston, then attending BU will be an advantage. If you want to match elsewhere in the country, I don't think UF puts you at a disadvantage vs. attending BU. Where and what you match into will be more a consequence of the work you put into the next four years.
For me, cost is going to ultimately decide where I attend. For you, if you cannot identify a single quantitative factor that will guide your decision, then I recommend going with your gut!