Some comparisons here:
https://www-secure.symantec.com/connect/articles/differences-between-symantec-endpoint-protection-11x-client-and-sep-121-beta-client
Mainly new additions with download insight and SONAR, better performance as well.
https://www-secure.symantec.com/connect/articles/difference-between-symantec-endpoint-protection-121-symantec-endpoint-protection-sbe-2013-a