How many of the cross-platform DAWs offer free versions of the opposing platform, like Sonar was going to be?
 
If I buy the Windows version, do I also get the Mac version or is that a separate purchase?

I have both Studio One Professional 3.5 (purchased after the Sonar announcement).. and I have had Pro Tools 12.x for over a year that came with an Eleven Rack I bought.. both DAW's have installers available for Mac and Windows once you register. 
 
So if you want to go Mac now that Sonar is no longer creating software.. you can finally cut the cord. I'm looking to do the same but have been "playing" with Studio One and PT (and actually Mixbus 4 which is surprisingly GREAT.. and has Windows/Mac AND LINUX versions.. =).. and once I finalize a "mac build" I'm going to Mac OS and running the DAW I end up liking most. 
 
(I also downloaded the Mac beta for Sonar to check it out). =)

Some of the mac DAWs arent ready for the latest mac OS I've been reading so I may wait to do a change over.

Reaper license is not bound to particular platform.
 
There is some similarity in Mac building routes with Sonar - both created it on Windows and then expanded to Mac. But they took different parallel roads. Reaper is based on own WDL (https://www.cockos.com/wdl/), so they have wrapped Windows specific code themselves. That works.
Sonar has delegated the task to generic Windows code wrapper, Codewavers. That was also a kind of working, but you know the result...
Funny that Reaper (windows version) perfectly works under Codewavers (Wine in Linux), without any modifications and without glitches (unlike Sonar X2/X3 which also work there, but with a long list of problems).