I create all my files for Live in SONAR (and most of my files for Traktor in Live ). So I'm really just bringing over loops and one-shots I've created in SONAR to take advantage of Live's performance-oriented audio engine and ability to switch scenes rapidly, as well as take advantage of Live-specific control surfaces.
 
Although the two programs appear similar on the surface, to me Live is a software version of a musical instrument and SONAR is a software version of a world-class studio. So now that I think about it, it makes sense I create the material in SONAR, then perform that material in Live.

@toddsilva
 
As Live 9 intro is bundled with a ton of gear, you can get a Live 9 intro license for cheap on Ebay.. i think i got mine for $15 or thereabouts... So before you delve into Live, get the current version... and at least a $99 Live controller as without it (as Craig states) its kludgy.
 
My initial assessment is that Sonar's Matrix view is a poor shadow of Live's session view,  Live handles 2-way communication with controllers better than Sonar imo, and does a few other things simply better.. otoh, things you take for granted in Sonar (multi monitors for example (and i use 3), only came to Live in v9 (2 screens)
 
For performing Live, or simply getting stuff ideas down, i suspect the consensus is that Live is better by a margin, but for straight up recording, mixing & polishing, its Sonar, again, by a margin.
 
I just hope that CW puts some effort into improving 2-way communication with control surfaces, and improves Matrix view.. then to me, Live would have little appeal, and Sonar would dramatically improve its market appeal.
 
I wouldn't recommend going down the path of Maschine.. mine has sat unused for some time.

Thanks Mystic38, helpful feedback. It's been a year since the original post, and I have since scrapped Live and started using Bitwig. So now using it along with Sonar. Really liking Bitwig, as it appears to me to merge both of the worlds, and fairly well given that it is still at version 1.x.

I use Live 9 standard and SONAR LTU.
 
I prefer Live 9's Warp and Groove pool to Sonar's Audiosnap.
But Live 9 could not use VST3 plugins.