Hi,
Both are beautiful, and it depends on what you are looking for:
-Sapphire Beach is part of a resort. You are allowed to use the beach without a problem. There is a marina, so if you wish to parasail or jetski, you can rent there. There are no changing facilities to speak of, other than the rest rooms.
-Magen's Bay is less commercial and more wooded. There are more areas to just relax. Even so, there are refreshment stands and changing stations.
I'm sure you will enjoy either.