congrats, Starsinthesky, for getting both offers!
I'd recommend the NIH MSRP over DD for the following reasons:
1. NIH offers a significantly higher compensation and provides all the lab supplies that you might need
2. dedicated researchers at NIH will most likely offer better mentorship than clinicians who do research on the side
3. you won't be able to get the NIH experience later in your career, whereas clinical research at a major medical center will most likely become a permanent part of your career.
4. Bethesda is likely a much more fun and comfortable place to be than most the DD sites.
I would have to disagree with you with one qualifier, that each person has their own requirements and goals from a year off.
1. I don't think compensation matters much in the scheme of things. Sure it might be a little extra that is nice to have, but you aren't taking the year off to make money. You are doing it for the experience. In addition, any PI that the DD institutions will approve you to work with will not have any funding issues. So lab supplies is also not an issue. They won't put you with some new PI trying to fund his project. In addition, my DD institution had extra funds for lab expenses if needed (for reagents, funds to cover research imagining studies...etc.). No issue with that. You aren't doing your "own" project anyways, but rather one that is ongoing with your mentor. They will already have funds for it.
2. If you go to a DD institution with strong research that is not a problem. My mentor has ~9 weeks of clinical duty per year. Yes 9 weeks. The rest of it he was in the lab/going to conferences/giving guest lectures...etc. He was around plenty for me to get his help and opinion when I needed it. In real life, you don't need your mentor around the lab every day. You want to have easy access (in person or by email if out of town) so that you can ask questions if something comes up. But ideally you want to at least meet in person 1-2 times per week to update regarding the project and any changes that need to be made. Truly successful mentor's know how to mentor medical students.
3. As others have said you can get the NIH experience later in life. Also I'm not sure why that is such a big deal. Depending on your field, NIH might not be the best location to get good experience in your research interest. I got accepted to the Cloisters program but eventually turned it down for a DD spot; primarily because the DD program offered a much richer research experience in my field of interest and I had already built a relationship with my mentor rather than having to seek out one at the NIH and lose precious time finding a mentor and then figuring out a project that would work within my interests and time frame. With DD, I arrived already running. Project was already approved by the IRB before I got there and I was enrolling patients my first week.
4. Location is very subjective. Personally I don't care for D.C./Bathesda but I am easy going and could live anywhere. If the best research lab/institution for my interests was in Alaska, I could tough it out for the experience. But I wouldn't consider the location as a draw for NIH. It isn't terrible but it isn't great either.