For an ideal case these will be the same, and the isolation will only be determined by the quality of the external match.
For instance, if you feed 1 and 2, you expect to see sum and difference at 3 and 4. If loads connected to 3 and 4 have finite return loss then some of the signal will get reflected back into the magic tee and show up at 1 and 2. The paper below shows how to calculate the isolation for this case (see table 3). However, there are many ways of implementing a magic tee in practice, and unintended coupling paths between circuit elements may decrease this isolation.