I thought I'd throw in my 2 cents on this since I currently have both the SR12 and the HG21.
To put things in perspective, I've had the SR12 since it came out and I had also tested the HF10 later on. My testing, which always consists of doing A/Bs on the same scene at the same time, showed what was IMO, better color on the SR12. It seemed that the color on the SR12 was more consistently accurate to the very blue leaning color of the HF10. Although the 'apparent' sharpness of the HF10 was a bit better at times than the SR12, there was no discernible increase in actual detail that I could see. So I kept the SR12 and returned the HF10.
Last week I tried the HG21 and immediately saw a big difference relative to my SR12. The HG21 has both greater apparent sharpness (significantly so at times) and a definite increase in actual detail. At times the difference can be truly striking...enough so that it actually makes some of my SR12 A/Bs look as if there was somewhat of a 'haze' on the SR12 footage!
The color on the HG21 also appears more accurate at times, if not a bit cooler than the somewhat warmer SR12 footage. Although I really think the HG21 footage is more accurate, I could see some people preferring the SR12's warmer colors.
I was doing an A/B in our kitchen and showed the results to my wife, not telling her which camera was which. My wife's usual response to these A/Bs are "I don't know, they both look good". This time, when the HG21 footage came on, her response was "WOW!". Just to make sure, I asked her what did she mean. She said the 2nd clip (HG21) was much sharper, brighter and had better colors.
Autofocus on the HG21 is superior to the SR12 as all the Canons tend to be. This is not to say I had any real issues with the SR12 focusing, but the Canon is simply quicker and its accuracy extends much further down as lighting diminishes. I often have hunting issues with the Sony as the lighting gets worse.
On the plus side for the SR12, I think its sound is better than the HG21 with the onboard mikes.
My A/Bs have ended and I'm keeping the HG21 and putting the SR12 on Ebay.
Don't get me wrong, the SR12 is still a great camera, but I really think the HG21 footage is superior.
Just a note on the networks and their choices in cameras. I don't believe that Canon is actually in the studio camera
business (I could be wrong), but they are most certainly in the high-end studio and professional field lens business. You'll often find Canon lenses on Sony camera guts. I almost always see the Canon lens on broadcast cameras at any ballpark I've gone to. I think this is due to the fact that Canon does make a better lens than Sony. The sourced Zeis lens for the consumer Sonys are not as good as the Canon lens on their consumer cams. As I was doing my testing, I often wondered if the disparity I was seeing was due to the lens or better imaging chips & electronics in the Canon. My gut is that much of it is the lens. Although there are times that Canon lenses exhibit a bit of chromatic aberration, the HG21 seems to be pretty much free of that. I haven't seen any significant purple fringing that often alerts you to this issue.