I love Sonar but there are two things I really miss from Ableton

P-Theory
But don't get me wrong there are some features in Sonar that Ableton doesn't even get close to.  As a traditional "tape recorder / Studio / editor" Sonar is second to non in my opinion, but for fast creation on the fly Ableton beats it hands down

This is why I use SONAR in the studio, and Live for live performance (although I do use SONAR as a plug-in host for live guitar playing). But creation speed also depends on the musical genre you're creating - i.e., whether you're trying to create a soundtrack, song, or beat. There are a lot of ways to do fast creation on the fly with SONAR, albeit not using the same techniques as Live. If you're ever somewhere I'm doing my "Recording on the Fast Track" seminar, which is based on SONAR, check it out; for writing songs where I play multiple instruments, I've yet to find anything faster than SONAR. 

I don't think Session Drummer is intended to accomplish the same task; its design goal is to be an overachieving metronome that improves on the original Session Drummer 1 and 2. As noted, there are plenty of 3rd party alternatives that do what you can do in Ableton, which offers the opportunity to choose an instrument that fits your need exactly. Let's give a big tip of the hat to Steinberg for creating a plug-in standard so DAWs aren't limited to the capabilities with which they ship 
 

P-Theory
Wouldn't it be wonderful if Sonar could do the warping and triggering thing as well as the excellent things it does already

 
Yes, but then reality enters the picture...I'm going to speculate that the Complex Warping algorithm is licensed (maybe from zplane?). Having that capability is essential to a beats-oriented program like Live. SONAR licenses some elements essential to its "mission statement" (e.g., Melodyne, AD2, iZotope Radius algorithm). If more elements were licensed for SONAR, the price would need to increase.
 
Ableton would be in a similar position re: pitch correction. If someone in the Ableton forum said "I love Live, but SONAR's pitch correction capabilities are a million miles ahead of Live," I'd point out that adding Melodyne would likely increase Live's cost, and there are third-party options for pitch correction. There are also third-party Drum Replacer options for Live.
 
Warping is Live's specialty, so it's worth paying the price needed to have that degree of warping refinement within the program. But, it's important not to overlook what SONAR can do. Acidization is very powerful, and SONAR is the only program other than Acid Pro that lets you create, edit, and optimize stretchable files with that degree of sophistication. Because of all the audio-for-video work I do, it's one of the main reasons I use SONAR. And despite AudioSnap's rough edges that requires more "manual labor," it does do the job for those who require customizable warping. I've climbed its learning curve, and get good results from it (particularly on individual tracks).
 
Finally, remember you can ReWire Live into SONAR, which I did for years before the Matrix View was introduced. Then you have a new program called SONive, or maybe LiNAR