MonoDevelop was forked from SharpDevelop but they diverged. Unity has MonoDevelop with Unity-specific patches and they are a little behind. Xamarin Studio is also MonoDevelop with proprietary stuff.
Speaking of the "linux version", linux distributions are very far behind in the package versions provided and the folks at Xamarin are only doing packages for OpenSuse. If you want to do development with Mono, Linux is unfortunately not a good OS to be on. Mind you, this is nobody's fault - it's just lack of interest.