rfreeze
My past experience with Sonar was that it was always way more sensitive to hardware than any of the others I tried along the way.  This is likely the case now.

I may be completely wrong about this, but I wonder if cross-platform operation can result in DAWs that are more "forgiving" because they need to work with a wider range of variables. Or not. Hey, I don't code this stuff, I just use it.

Anderton
If you have the bucks, RME is about as good as it gets for latency. I believe Noel Borthwick uses the PCI version so I guess that's some kind of Cakewalk recommendation  
 
For USB, I'm currently testing the TASCAM US-4x4 for suitability with amp sims for live performance with guitar. I'm getting a reported 4.3 ms RTL at 96 kHz, which is holding up even with multiple sim instances (e.g., one for each string). 

That's a good recommendation.  I'll write that one down  :)

Anderton

rfreeze
My past experience with Sonar was that it was always way more sensitive to hardware than any of the others I tried along the way.  This is likely the case now.

I may be completely wrong about this, but I wonder if cross-platform operation can result in DAWs that are more "forgiving" because they need to work with a wider range of variables. Or not. Hey, I don't code this stuff, I just use it.

I think you're right.  There's an unavoidable layer of abstraction that pulls them back from the weeds.  And at least one that I know of that manages to do this while maintaining awesome levels of audio engine performance.

Delta66 with Win7x64 drivers and X3e Prod - Win8.1x64 system
 
UPDATE:  I reverted back to an earlier Win7 driver release after seeing a forum post somewhere that the prior release was more stable with 8.1.
 
With a 128 sample buffer @ 48K (6.8ms rt latency) X3e has been rock solid.  I just finished mixing a collab project that is heavier than I typical have. 22 audio tracks with a healthy assortment of compressors, reverbs and a delay or two.  Sonar basically idled its way thru without so much as a glitch anywhere.
 
I'm sooooooo glad my Delta is still kicking (ass)

Only just spotted this...
 
A couple of months ago I plugged in an old Delta44 becasue I was tracking down whether there was a a fault in my then interface (UA-101) or a Windows problem.
 
Neither of the M-Audio Win 7 drivers worked for me under Win 8.1. The later driver simply resulted in Windows blue screens which indicated the Delta driver was to blame, or no audio at all, not even MEdia Player.
 
The pre-SP1 driver would give me solid, basic playback, but recoerding was the opposite of stable.
 
I guess it's the standard PC problem that there are so many possible hardware variants and combinations that it's no surprise that once in a while weird things happen. Apple don't have this problem because they are always working with known hardware. For which they charge a great deal more money than you'd pay for a similarly specced PC.