The ivy name really doesn't carry the same weight at this level as it does for undergrad. It's one thing to choose UMass over somewhere like Columbia or UPenn, which are in the top med rankings and are powerhouse medical schools. But Dartmouth is similarly ranked and will likely have similar opportunities for furthering your goals, so I would not get too caught up with prestige/reputation for this particular decision.
If you have high reaching goals e.g. uber competitive specialty or academics, then Dartmouth probably has a very slight edge. But in the end, to even be competitive for those types of roles, you will have to do very well throughout the 4 years. So, if fitting better with the student body + being near your SO and friends + graduating with less debt are things that you will need to enable you to stay motivated and sustain your happiness, then UMass is the obvious choice. I would say that most people fall into this category from what I have observed, but perhaps your priorities are different.