Q: Whats better Mountain Lion or Yosemity

Yosemite is better, but unless you downloaded it previously it is not available. Install ElCapitan

ElCapitan

Instructions for upgrading to El Capitan >  Upgrade to OS X El Capitan

As with any upgrade, make sure to backup all your important data prior to upgrading.

I am, actually, very partial toward OS X 10.8 Mountain Lion and OS X 10.9 Mavericks.

I was using, testing things out in OS X 10.9, but some of my older software that still worked on OS X 10.9 Mavericks was glitchy and crashed more often.

I have sort of "rolled back" , for the time being, to OS X 10.8 Mountain Lion and it seems most of my older software is running more stable on it.

I miss some of my new software and newer software updates on OS X 10.9.5 Mavericks, though.

My opinion here, El Capitán may run better and have some great new features, but, like my older iDevices, my older 2009 iMac, doesn't take advantage of all of the new feature set of Yosemite or El Capitán and, again, IMSO, the UI is PlaySkool ugly as hill.

If the computer is new, it can't run an OS earlier that it shipped with.

Install OSX Restrictions