After I rescan the collection, nothing shows under the collection tab. It worked with the svn version, but doesn't with the normal version. I have the codecs and stuff, because I'm playing songs through Amarok2 now.

Downgrading Amarok is generally not officially supported (actually most apps don't really support it). What can happen is that this will confuse the config and/or database.

Make a backup of the following files, and then delete them:

$HOME/.kde/share/apps/amarok/*
$HOME/.kde/share/config/amarok*

the svn version is the newer version? Is that the usual case for all applications with svn, git, etc?

Naturally the SVN version is newer - it's constantly being updated! As soon as there's a release of Amarok (in the latest case, 2.0.2) it's technically out of date compared to any current SVN checkout. Bear in mind that the tarball that was packaged for 2.0.2 was created a week or so ago for testing purposes and to give the distros time to roll their binaries!