Audit Requirement - Capture Logins vs Database
For this I would default to extended events for the 2008 server and up. There is a good blog post about it here. 

Additionally, for the 2005 server, you would need to use a trace as the extended event functionality wasn't available. There was another post for this shown here.

Both of these options give you different ways of storing and what to capture such as login name, time, database name, Etc.