I don't want to discuss "what is better" in general, because everyone has its own preferences, and for live purposes Ableton for now is the one and only.
I can't discuss about quality of suming, about overall ergonomy as an effect of totally different plilodophy and DNA.
but I can say few things. Love it or hate it, but in many cases Sonar is much better than Ableton, but in many important details - Ablton has no competitors. Even Sonar is just a point somewere in rear mirror.
And - Ableton together with Sonar, could be great duet. Sonar for scoring/composing, mastering, Ableton for sketches, fast pattern-based music, basic recording, tweaking and live performance.
But if you think that Sonar's Matrix will replace Ableton at all - you're wrong, because Ableton is much more than just "matrix" - it's all about MIDI, controlling, live tweaking, MIDI mastercontrols in channelstrips and in softsynth/FX-strips....
/Sonar has only (ONLY) ACT, which was maybe nice tool 5-7 years ago, but today it's just an epic fail./
Maybe these things are not as useful in studio environment - but just "maybe". Try Ableton, STONE STUDIO, befor you trash it. Or give me example, how to use "stupid" Guitar Rig's live mode in Sonar. Or how to map few controls to FXstrip, which includes (i.e.) 5 different FX.
In a fact I almost love Sonar, but I won't leave Ableton. I will do it just in one case (it gonna be cold day in hell) - when SONAR get the same MIDI functionality and ergonomy.