Usually SourceForge has the latest stable release of Lazarus. But after a while Ubuntu repository will catch up too.
The packages on SourceForge and Ubuntu repository are not exactly identical, combining them usually will cause problem. And that is one of the reasons to fail if you try to install Lazarus when using latest version downloaded from SourceForge. It (the Ubuntu) may 'force' you use the FPC in the Ubuntu repository. Not always but some in my tests, I had to disconnect my internet connection or it won't able to install successfully.
And of course, if the version of Lazarus you wish to install is already in the Ubuntu repository, using it is the easiest way to install Lazarus.