It's not a naive question! Normally, this is a case where you should just take the guaranteed MD acceptance and run, but hear me out.
I had the choice between a BS/MD program (as well as an early assurance program which began my sophomore year of College) and another school where I saw myself being happier. My HS advisor (bless her soul) gave me this solid advice which I'll paraphrase: Getting into a BS/MD program is amazing, you're already an outstanding applicant. If you keep that same drive you will have no problem getting into a regular MD program because it's already more difficult getting into the BS/MD/Early Assurance programs. Go where you think you'll be the happiest and feel the most supported because that's where you'll shine.
Now medical school apps weren't this competitive and crazy back in 2016 so I chose the school where I'd be happier. In the end, that gut feeling was correct, I fell in love my school. The support I had there (friends, family, faculty, etc...) really made the grind that is pre-med so much more manageable. And it wasn't even perfect, over the next 5 years I fell in and out of love with medicine a couple times, explored other avenues for a bit, but eventually found my way back to medicine (I ended up having a crazy successful cycle and will be attending a T5 this fall so no regrets on making that decision)!
That said, Pitt is an amazing school with an even more amazing medical school. Objectively, if this was me, I would take the offer and run (you have a guaranteed acceptance to Pitt, that's actually WILD!!) with it because lord knows how competitive medical school applications are gonna be 4-5 years from now. BUT, if you truly see yourself happy at Cornell, take the happiness route because mental wellbeing is criminally underrated and can make or break your undergrad career. Cornell will set you up just as well for medical school, you just have to keep that same drive you had to get accepted to the BS/MD program (Given that you got into Pitt's BS/MD program I'm sure this won't be an issue for you). Additionally if you choose to pursue a different field the Ivy name carries weight and will provide you with so many connections.
Anyway, just my two cents! Feel free to PM me if you have any other questions!