Whats the difference with session_api?

I thought the difference was clearly explained on the project page. See the FAQ section below copied from the project page.

No there is no conflict when both are used.

PLEASE NOTE: both are API's so only do something if a developer of another module decides to call the functions in the API. If you switch either or both modules on in a vanilla D7 core install, then NOTHING will happen. Session API doesn't do anything until you switch the Flags module on (or one of the other modules that use it). Session Cache API does nothing until you switch on Smart IP, IP Geolocation Views & Maps or Views GLobal Filter.

FAQ's

Q: What is the difference between this module and the Session API module?
A: Session API creates alternative session ids that remain valid after authenticated users log out and become anonymous. Session API does not implement any session data storage mechanisms, whereas that's what Session Cache API is all about. Session Cache API sessions also continue to live after logout, so you normally don't need Session API when you already use Session Cache API. See the README for more info on sessions, Session API and Session Cache API.

Ok. Closing.