The value-add is that the MAG is the "go-forward" platform. Code is pretty much identical except that there are a couple of features that are not offered on the MAG such as IVS as Juniper has a different approach to offering that type of solution.
The SAX000 series are EOL. The SAX500 series are still sold but as a reseller I can tell you we don't sell them except, as Zanyterp said - to legacy customers for compatability for things like clustering.
The MAG is newer, better form factor, etc. . .