The lower divs are all the same for gen bio and MCD so your first two years aren't going to make a difference.
The biggest difference is that general bio requires only 1 molec/cell bio class as well as 1 ecology/evolution and 1 physiology class, whereas MCD doesn't require ecology/evol or phys and does require both Cell Bio and Eukaryotic.
In majors there are usually a few 'barrier' type classes, the ones that sort the kids who really want the major from the ones who just didn't know what else to do. Organic chem is the first barrier and I knew a bunch of gen bio/MCD kids who changed to marine bio/EEB because they couldn't pass it. The next barrier class is Eukaryotic. It's required for MCD and not for gen bio (it's a possible elective). Knew quite a few people who dropped Euk and changed to gen bio as well. A lot of gen bio kids take human physiology which is the hardest gen bio class, but it's not absolutely required for gen bio, it's an elective.
You can see all the required classes here
to compare/contrast. Ochem, Euk and Physiology were known as the hard ones.
All of these are really big lecture classes, it's the labs that you're going to have a hard time getting into.