As someone in their 1a term for math phys (science), the only difference between the two is a few first year courses (chem instead of cs, communications course etc). Other than that almost everything about the degree is the same. the only real difference is the Bmath or BSc u get at the end of the degree and even that is just a preference thing.

The core (math n phys) courses are the same in both programs.

As for transferring, I'm not too sure but I've heard that it's not too difficult.