I use both PT and Sonar.  I prefer Sonar's workflow and virtual instruments when I am writing and recording myself.  If I were to track a full band, I would much rather track/mix in Pro Tools.
 
For any audio post production, Pro Tools is light years ahead of Sonar.  Pro Tools is a more advanced tracking, editing and mixing software.  Sonar does not offer the advanced key binds, navigation and editing features that Pro Tools does, thus making Pro Tools a much more efficient audio editor. Pro Tools midi used to suck but in the last few years has really made leaps and bounds in this area.  I still prefer Sonar for writing orchestral mock ups, but could definitely use Pro Tools of I had to.....and it looks like I might :( 
 
Both are great programs with different strengths.  Coming from Sonar originally, it took me quite a while to wrap my head around the bus layout of Pro Tools.  Routing might be your biggest learning curve if you were to switch.  Pro Tools is a very deep program with tons of options.  If you are a solo bedroom recording musician than almost any modern DAW will work just as good for you, but if you are a professional audio editor, engineer or tracking live bands, Pro Tools will win hands down in the workflow efficiency category.   
 
 

I was planning on replacing my DAW hardware next year anyway.  Given the Sonar news, I'll use hardware replacement as an opportunity to migrate to new DAW software as well. 
 
Upon reading comments for quite a long time comparing Sonar vs. PT, it seems both platforms have their strengths.  Using a massive sweeping generalization, Pro Tools seems to get favorable reviews by people who are primarily working with recording / editing digital audio and concerned about interoperability with many high-end / pro studios.  PT tends to get poor feedback from those wanting more advanced functionality and working with heavy MIDI / sequencer focus. 
 
Some may want a DAW that's most similar to Sonar to minimize migration learning curve.  I don't earn my living recording music, 95 % of my DAW time is spent on recording and editing digital audio (almost no MIDI / soft synths). 
 
A big frustration in recording my last album was I spent countless hours tracking, then mixing / editing in Sonar.  I hired a pro engineer to complete mixing for a professional end result.  The Pro engineer used PT (as most do), so I had to export everything to OMF (or gave him WAVs of some items) losing many of the mixing details I put countless hours into (panning, etc. ) 
 
I will follow the same path of tracking myself and hiring a pro to complete mixing - interoperability is key.  So I will be evaluating PT closely when selecting new DAW software, as I do not want to repeat the situation above 
 
Hope that helps.
Russ

I once tried a version of ProTools that came with hardware I bought. 
 
I wasn't overly impressed. It seemed totally geared for audio production, and my work is mainly MIDI-to-audio. 
 
And performance on my (at the time) state of the art PC was sluggish as hell. 
 
I wouldn't serious consider it today, especially since I don't want to be locked into a proprietary plug in format and I absolutely refuse to use any software that uses an ILok. 
 
Sonar Platinum does everything I need it to do. As long as the servers keep running and it works Windows I have no desire or need to ever switch to anything else.