Congrats on both of your acceptances!
I went to Mills, so please feel free to ask me any questions you have about the program. It's a wonderful program, and I absolutely loved the professors and students.
If Northwestern does have a linkage program now (esp. with itself) then that might be a good reason to choose it over Mills, assuming you're interested in Northwestern for med school in the first place. Northwestern's program will probably also be better known in the Midwest than Mills. I interviewed at Loyola, and they weren't too familiar with Mills. I guess it's possible schools may not know much about the NW post-bac program itself, but at the very least, most, if not all, medical schools will know the Northwestern name, as it's a prestigious school.
Mills is great if you want to go to a UC, though you'd probably be considered an out of state applicant, so I don't know how that would affect you.
Taking two years to complete a post-bac program is a great idea for those who weren't science majors--this is part of why Mills students do so well, because it gives them enough time to really learn the material, and still have time to do EC's. It's pretty difficult for those new to the sciences to finish a post-bac program in one year and still have time for outside activities. I know most post-bacs want to get to medical school as soon as possible, but the most important thing is to get there, and it helps to take your time to be the strongest applicant you can be.
With that said, you can actually finish Mills in one year, if you've taken general chemistry (if you haven't, you can take gen chem in a number of intensive summer programs, at least in the SF area). You then take bio, physics, and o-chem (the "triple-threat" together. It isn't recommended, but I decided to take this route, as did two other students in my year. We all did really well, and still had time for EC's. However, I knew others who tried this the following year and felt it was overwhelming, and decided to drop a course and take two years to finish the program instead. If you're at all interested in this route, I'd be happy to give you more details about it.