I was choosing between the same two programs - it's a tough choice to make. They're both great programs (so many you can't go wrong!), but obviously I can only really share my experience at Tufts. To answer your questions...
1. I can't remember the exact number of students in our class but I do think it's around 120. We are not broken up into smaller groups. Our main lectures take place in one lecture hall with all of us together. Occasionally classes have "conferences" which are when you meet with a much smaller group of students to discuss a topic that has been challenging for students to grasp in the past. This is the same as how the medical students have their classes. It might seem like a lot of students but often it doesn't feel like it. First because some students don't go to class since all of them are video-recorded, and second because the professors are often willing to take questions during class and always stay afterwards to answer questions.
2. Grading is scaled. For example, I think an 88-94 is an A whereas that would normally be a B+/A-. It's the same for every class and I believe it's the same scale as the medical students, though they are also pass/fail and we are not. Note that this is NOT a curve. You are NOT being graded against your peers in this program, which I absolutely love because it means we aren't competing with one another. This is something the MBS staff really emphasize and the students, in my experience, really take to heart. We ALL help each other and share resources. As far as difficulty, that's hard for me to judge because it varies between people. There are students who fail tests, students who get 4.0s, and a bunch of people in between. The program provides us with a lot of resources to support our learning. So far, I haven't found the material any more difficult than in undergrad, but the AMOUNT of material is much greater. Honestly I think 99% of this program is how much time and effort you're willing to put in because the professors, TAs, and other students are really willing to help you understand any concepts you find challenging.
3. You are assigned an advisor when you start the program and will meet with that advisor at least a few times during the year, if not many more (up to you). Advising kind of depends on who you get. I have some friends who love their advisor, and others who are kind of so-so about them. However, several professors literally say during class that they don't want you to ever feel like you haven't been "advised" enough while in this program and they are absolutely willing to meet with you to give a second opinion on anything. I've had a pretty good experience with my advisor, but I also reached out to one of my professors for advice on something. It's not perfect but I haven't felt like there wasn't anyone to ask for advice from.
4. There are a lot of required classes! All but 1-2 classes are core classes. You won't be taking more than 4 courses at one time, but they are blocked like the medical students. So for example, I took 9 total classes last semester, but only 4 maximum at one time.
5. Boston_med would probably be better at answering this question. I'm not really the best with MCAT advice, honestly. I took the GRE before entering the program. You will have the option to take an MCAT class during the Spring semester (ends in late May) or right after classes (probably goes to mid-July). You have to way some pros and cons. If your grades are stellar first semester and you want the MCAT over with ASAP then you might take the Spring MCAT course if you're willing to let your grade potentially fall a little. Flipslide is taking the summer class and focusing on your grades in the Spring, but then you take the MCAT a little later. I have heard from friends that took the MCAT before entering the program (it seems like most students did) that it would've been helpful to have had the MBS classes before taking it, especially those that didn't major in biology/biochem.
6. Again, boston_med will probably be better at answering this one! I assume that they must be pretty successful because 87% get into medical school somewhere and 35% get into Tufts. Here's a list from their website of schools MBS students have been accepted to -
http://publichealth.tufts.edu/~/media/PHPD/PHPD PDFs/Academics/Professional Schools MBS Students Have Been Accepted To.pdf