The move to Cubase from Sonar has been smooth for me, steep learning curve at first, but then once you dive in, you realize it does things in a similar way to Sonar, just slightly different terminology.
No more Staff View or AudioSnap blues for me.
It's a shame Sonar ceased right after they perfected their Piano Roll View . . . the Cubase PRV is good, but not as good as what the Sonar bakers came up with.
I've even been able to drag and drop midi and audio files from Sonar Platinum right into Cubase Pro, just one at a time, but still amazing.
Sure Cubase Pro version is priced higher than some of the others, but it is an established solid DAW environment.
It probably has the most advanced features available for more traditional music composers.
If your DAW usage is geared towards live music, beat looping, EDM, or just audio mixing . . . Cubase Pro may be overkill, as many other cheaper DAWs do a great job for those. (I've demo'ed many in the last few months)
For sure check out the crossgrade offer on the Steinberg site, or is that $600 AUD ? . . . not sure on the conversion.
Extra bonus is that the Cubase forum mentions ARA technology is on their front burner, and I believe Steinberg has the development resources in place to do that easily.