Hi Pietro,
Change Doc Activation different from Log EH Posting.
Change Doc Activation - This is gives Date, time, parameter, old value and new value, table etc that shows what changes has happened but will not give you in detail how it has happened .
You can find this in /SAPTRX/EH_LIST -> Event Handler Detail Overview -> Change Documents.
Log EH Posting", "Log Msg.Proc." and "Log exr.Updates - Gives you detail step by step how and what has happened during posting. If you enable you can find the log in Tcode SLG1 and /SAPTRX/EH_LIST -> Display Event Handler Log(F7).
If you are posting less than 25000 events per day, it's ok to activate, however if high number of events posting then ensure that you have strong system resource planned to mitigate the risk.
Regards
GGOPII