I'm a Sonar user as well, or I was. I'd been using Cakewalk stuff since Pro Audio 9. But since about Samp 10.0 I stopped using Sonar for much. I stopped at Sonar 6, didn't bother to upgrade to Sonar 7 or 8. I just haven't needed Sonar in any significant way, even for midi (Sonar still has a feature edge here but for all the basic stuff Samp is more than capable now and the editor just keeps getting better and better). That's not a knock against Sonar, it's just that Samp does everything I need now and I prefer working in one application whenever possible.
I worked in Sonar several years before trying Samp and when I first demoed Samp I was instantly struck by the sound quality. I purchased Samp as a crossgrade a week later. That was back in '03. To this day Samp still sounds better. I know that's a highly debatable, subjective issue but that's my reality. I later switched to Samp for rent, economical and free upgrades. If your hesitant about the initial costs to migrate to Samp you may want to consider the Samp for rent option, quite painless.
As for performance and stability Samp has always been at least as stable as Sonar if not more so. If your system is functioning well already you should have little or no issues with Samp. I used an Athlon 64 x2 4600, 2Gb Ram, Lynx2 interface on WinXP SP2 and SP3 for several years with both Sonar and Samp. Samp ran very well on this platform. You didn't specify your motherboard but I've been using Gigabyte boards and they've been very stable. I just last week upgraded my entire system, new motherboard (Gigabyte again), new cpu, new heatsink, new RAM, new video card, new PS and after updating a couple of drivers I ran Samp and it behaved as if nothing had changed, except it was using about half the cpu it was using before.
I'm not a business/production user of Samp, I do mainly my own stuff and tracking, mixing, quasi-mastersing for friends and locals. But I do push my system and Samp hard at times. I use UAD-1s and UAD2, drum tools like Superior 2, Addictive Drums, synths like Stylus, Atmos/OmniSphere, Trilogy, Kontakt, etc. and anywhere from 10 to 60 tracks with numerous aux. I do all my projects at 96k/32bit and on many projects I'm right up to the edge in cpu usage, 85-90%, and Samp still keeps ticking. If I worked at 44.1k Samp could probably do my stuff with one hand tied behind its back. I have used the hardware insert function a few times and it has worked for me, although on a few occasions I've had to slide tracks a bit for perfect synch but it does work more or less as advertised. Definitely better than Sonar's implementation which I tried a couple of times and then left it for dead.
With a well functioning system you shouldn't have to spend too much time getting basic things going like recording/playback, editing, synths, mixing, etc. but there is definitely a learning curve to Samp and all of its features and tools.
Samp's implementation of object level functions and tools is one of its biggest advantages. It just has a great workflow and once you get the hang of it I think you'll find it both speedy and logical.
Although Samp doesn't come with quite as many addon features, synths, plugs, etc. as Sonar the extras that are included are mostly top notch. Sascha's AM plugs are first class. I haven't really played with Samp's addon synths or drum plugs as I don't need them.
On a subjective note I just greatly prefer Samp's interface. Whenever I need to open Sonar to migrate old projects into Samp I always cringe for a moment when I see the track view and the console view. I know there's plenty of options and power under Sonar's interface but I have never really warmed to the visuals and the layout. Samp has some excellent skins available done by third parties by the way.
Now I don't work like Bob Katz and I'm certain he doesn't use my workflow So I can only speak for my experience with my gear. As with all software Samp has things that should be fixed, things that should work better, features that should be added. I have found Sonar to be mostly the same in this regard except Sonar seems to have more items that should work better. Samp will need to be configured for you specific environment and hardware, things like ASIO settings, buffer settings, VST settings, etc. will need to be optimized for you and the way you work or you may find the default settings work great. Samp has a lot more flexibility when it comes to system configuration. And, of couse, you'll have a great resource here at the Samp forums as well with a lot less "noise" than the Sonar forums Download the demo if you haven't already and check it out.
Let me know if you have any specific questions and I'll try to answer. I don't use all of Samp's features or tools so you may need to search the forums to find more info on specific functions, tools, bugs, issues, etc.
That's it, that's my sales pitch.