John
I am unsure which has a better sound, Sonar or Mixbus. It may be that they are both very good but different.
This true for me too.
I have a song that I unfortunately cannot share (NDA in force) that is an indie pop thing. Not my song so I am unbiased as to what I wanted out of it. I gave it my best effort in both SONAR (using Waves, UAD and LX480. Slate FG-X was on the master bus) and then in mixbus using MB plugs + LX480.
To my ears, MIxus 3 won this round.
Back in the MB2.5 days I did almost the same thing with a CHB song where I simply stripped all plugs in both SONAR and MB2.5 and only attempted to set levels identical.
When I A/B the two raw mixes (no plugs used in either DAW) MB2.5 won by a slight margin as I felt the result was a bit more transparent and clearer. Not by much mind you but just enough to say "hmmmm, I like that better".