OSX is indeed a real Unix... but the user world has moved on to Linux where things don't just work but are also easy to setup and control.
If you are used to a Linux install, going to OSX will be a shock. For one, there is no native package system, you will have to install one of several on your own. They are not nearly as reliable as say the debian system. It is do-able but for young people it pays to remind yourself that there is a reason UNIX never took off. That BSD never took off. Linux (the whole eco-system) did far more then make a Unix compatible system, it made Unix usable for the average geek. The "real" unixes were bastards to work on with each system just totally different enough to not make them at all compatible. Not like the way you can google a red hat fix and apply it to ubuntu or the way a arch-linux wiki page is useful to a gentoo user.
OSX made Unix usuable for the average hipster but crossing from geek Linux to once touched a girl OSX will be a shock, just how many things are different and just how much of OSX overrides the Unix way of doing things.
You can run FOSS software but it is NOT as easy as with a debian system. Before you buy a OSX machine to replace your ubuntu install, get an OSX user to show case their FOSS capabilities. Let them show you how they install an apache upgrade not yet released by Apple. Then go and hug your Ubuntu box and swear you will never ever look at an other system again.