Find the attach article
https://www-secure.symantec.com/connect/articles/few-differences-between-sep-11-enterprise-edition-and-sep-121-enterprise-edition
https://www-secure.symantec.com/connect/articles/feature-differences-between-sep-11x-and-sep-121
Re-Edit
Check the thread
https://www-secure.symantec.com/connect/forums/compate-between-11x-and-12x
https://www-secure.symantec.com/connect/forums/difference-between-scan-engines-11x-and-12x-sep
https://www-secure.symantec.com/connect/forums/difference-between-sep-11-and-sep-12
https://www-secure.symantec.com/connect/forums/sep-12-vs-sep-115-vs-sep-117
Symantec Knowledgebase Articles:
1) Symantec Endpoint Protection Getting Started Guide 12.1
http://www.symantec.com/docs/DOC4322
2) What's new in version 12.1
http://www.symantec.com/docs/HOWTO55189
3) What is new in Symantec Endpoint Protection 12.1?
http://www.symantec.com/docs/TECH163413
4) Planning for Migration to Symantec Endpoint Protection Manager 12.1
http://www.symantec.com/docs/TECH162730