As of this writing Cosmoteer is available on Steam as a demo. There should be a download link on the right.
Since the first Steam version, it offers "betas" branches for every stable update, which allows to revert the Steam version to an older version if needed. It is not possible to go back before the first Steam version.
The Steam version has multiplayer, and can play LAN or online. The early Steam version does not have "invite a friend" feature.
Note that using [code]--rootsavepath[/code] will break steam cloud. Since steam doesn't know that you're changing the save directory, it'll still sync the default directory.
Other than these the Steam version is the same as the normal version. The procedure to install and use a mod is the same by using the in-game Mod menu.