From the practical point the differences are that this package (mmjre) installs the Sun Embedded JRE (Java Runtime Environment) while the icedtea6 package installs basically the OpenJDK JRE and JDK (Java Development Kit).
So if you want to "just" run Java applications you can use any of these two.
If you want to develop Java applications on your N900 you'd have to go with icedtea6
Also note that Dak is working on a convenient way for launching downloaded applications from *.jar files. The icedtea6 package still requires you to start downloaded applications from the command line using "java -jar application.jar".

There are also some differences with respect to licensing e.g. the icedtea6 package is OpenSource; don't know which license the Sun based package is using.

There also might be differences with respect to which degree the different versions are optimized for use on embedded platforms.
What I can say for the icedtea6 package is that I currently uploaded two different versions.
The first one (icedtea6 1.5.0-7 in extras-testing) packages a version of the OpenJDK which was optimized for running on embedded platforms but it only features an interpreted mode (http://camswl.com/).
The second version (icedtea6 1.8-2 in extras-devel) simply repackages the OpenJDK packages from Debian for optification. This version also includes a Just In Time compiler (JIT).
Though, I didn't do any benchmarks so I cannot tell how these really compare performance-wise.