Hi,
SEP definitions remains same for all the versions so we can say protection level remains same & we do recommend to use the latest SEP version to get benefitted by new features and enhancements.
If you checked SEP 12.1 RU2 & RU6 fix notes, at the bottom you can see components & their respective version details.
RU6 fix notes: http://www.symantec.com/docs/TECH230558
RU2 fix notes: http://www.symantec.com/docs/TECH199676
If we talked about features, In most of the cases like 'Host Integrity' policy was introduced in SEP 12.1 RU5, unless client is on the same version policy can't be implemented. So to use all the latest features SEP client should have latest version.