Hello,
SEP 12.1 RU1 MP1 is a Maintainance Patch over SEP 12.1 RU1.
This Maintenance Patch has Lot of new Fixes and Lot of new Features, check these -
1) Release Notes for Symantec Endpoint Protection, Symantec Endpoint Protection Small Business Edition, and Symantec Network Access Control, version 12.1, Release Update 1 MP1
http://www.symantec.com/docs/DOC5564
2) New fixes and features in Symantec Endpoint Protection 12.1 Release Update 1 Maintenance Patch 1
http://www.symantec.com/docs/TECH187656
Now, it is upto you as an Administrator on how you want the Symantec Environment to be. In case if your environment is facing any issues which may have the resolution in this Patch may resolve the same. You could know the same by checking the Release Notes above.
It is Recommeneded and Advisable to have the Latest Version, however the choice is always yours.
Secondly, you are could also test these Latest Maintainance Patch on your Environment by installing the Patch on few of your Test Machines, which may provide you confidence.
Just to let you know that SEP 12.1 RU2 is on its way. SEP 12.1 RU2 is still under Beta Testing.
Secondly, incase you are interested in Beta Testing for SEP 12.1 RU2, check this :
https://symbeta.symantec.com/callout/?callid=1D5D3DD349C248E69B13977DA4DBF426
Hope that helps!!