Hello again FSCITADMIN,
actually there are 3 SME flavors!! Very fortunate community here.
SME 6.0.1 was the last final release;
SME 6.5RC1 is the next heir appearent it is however a release candidate(RC) and not finalized;
SME 7.0alpha6 is just what it says, an alpha, the early stages of development.
Both the SME 6 versions are Redhat 7.3 based. The (future)SME 7 is being built from CentOS base.
My primary website sits inside an SME 6.0.1 box. It has been exceptionally reliable. My future replacement site is being tested on the SME 6.5RC1 which also seems to be very stable.
Your risk/comfort level should be the determining factor on which one to use.
Have fun.
Joe