Now, i'm being told that this will terminate the session (or is it all sessions?) in the 15th minute of use, regardless their activity.
No, that's not true. The session-timeout
configures a per session timeout in case of inactivity.
Are these methods equivalent? Should I favour the web.xml config?
The setting in the web.xml is global, it applies to all sessions of a given context. Programatically, you can change this for a particular session.