I used sonar for about 2 years with my sblive, then when i wanted to get a good soundcard, I went with the digi001 so that I could use it with ASIO for sonar, and at the same time learn protools software, because its the industry standard.
Having used both, my opinion is that Sonar is much more powerful, easier to use (especially for editing), waaaaaaay less demanding on your system, and just all around better.
That being said, I'm sure there are lots of protools users that would say the same about sonar, and I'm also sure that the fact that I'm so used to sonar and protools is a lot different has a play in my opinion.
But yeah, thats what someone thats used both thinks.