Have you tried the trial? http://www.cakewalk.com/Products/SONAR/Web-Trial.aspx
  
I'm not sure, but I don't think Studio comes with the pro channel. For me being able to add an additional FX chain and whatever plug I want, has given me the freedom to be able to freeze a synth once I'm done and I can still work with it in the channel later on without having to reload that synth. That is my favorite addition. 
May not be something you are interested in if you rely mostly on 3rd party plugs but IMO the pro channel has some serious mixing tools, if it doesn't satisfy you for something with the FX chain insert you can build your own. I personally like that flexibility.
 
I would certainly DL the trial first. Some hate it and prefer Sonar 8

Transition from 8.3 to X2 will takes time, so much of the interface has changed that initally you will be totally lost so first impressions can be misleading. IMHO you should also get the SWA complete Sonar X2 video http://www.store.cakewalk.com/b2cgbp/ProductInfo.aspx?productid=30-SASX2.00-20E as this will really help.
 
The upgrade from 8.3 is well worth it. For me 8.3 to 8.5 was a major improvment and the X series built on that. I never warmed to X1 but added features in X2 really helped, like my favorite new feature, auto track zoom.
 
Pro Channel is good, and you will only get that if you get producer. If you never had it and have some decent effects you probably wont miss it but once you have it you will never want to loose it.
 
 

I really like the X series of Sonar and the transition was childs play for me. Probably the most important reason to get Producer version, if the instruments aren't important to you, is the Pro Channel. I really, really like the Pro Channel so that was an easy one for me. But core functionality is the same across all the versions as far as I know.

The pro channel is the deal breaker for me. I would find it very hard to go back to pre-ProChannel times.

Studio is fine if you don't want the PC - but I wouldn't want to be without it.

I found the change to X1 a sharp learning curve, but picked it up very quickly and loved it. X2 just improved on it with some great new features. Auto track zoom, take/automation lanes, console emulation and timeline zoom spring to mind.

Get Producer! Hands down, more bang for the buck. And it never hurts to have a viable alternative instrument or effect in the arsenal. While the instruments and fx in Producer may not be equal to one that costs several hundred dollars, they still can become a solid contender for a particular problem solving situation.

Sidroe - +1

If you hold out you may get a "Buy X2 Producer and get X3 FREE" deal...

That has got to be on the horizon....=P

cclarry
If you hold out you may get a "Buy X2 Producer and get X3 FREE" deal...

That has got to be on the horizon....=P

The deal ends Aug 31st. I can get Studio for $49!