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Compare them to Linux like this:
Solaris 10 3/05 is like kernel 2.4.22 which was an update to the stable 2.4 series.
Solaris Express is like Linux kernel 2.5.x, which was a dev release working towards 2.6.
If you are a user, you want Solaris 10, if you are a developer, you may be interested in SX.