I would have them both if I could. They both have strengths the other doesn't have.
Much of it depends on you and how you work. I was also happy with Sonar.
I seem to have happened into some of this. In the case of Studio One. I bought one of their interfaces a long time ago which had a copy of artist in the deal. I was signed up to their email specials. That software just sat and gathered dust until I seen a special deal and upgraded. From then on it was easier to upgrade. I kept upgrading it even though I didn't use it as much as Sonar.
A similar thing happened with Ableton. Version 4 was a great deal and I had it. Also sat on a shelf gathering dust until I needed to do some live work and seen Ableton had a deal going. For probably a third or less of the retail price I was able to upgrade each one...so I figure why not?
Ableton is mainly used by loopers and DJ's in what they call the session view. There must be 100 controllers that you can buy to use with session view. I mostly DON'T use the session view. I use tracks view much like a traditional daw. The nice thing about Ableton is you can do that. It is probably the easiest daw there is to map midi to. Push one button and map..simple. In that sense I'm probably not the typical Ableton user.
The other real asset to Ableton is the warping features. If you do a lot of key changes and pitch changes it would pay to look into warping.Still feels odd compared to Sonar. When I seriously compose acoustic instruments I use Sonar or Studio One. Then I might put those tracks into Ableton.