I just read your PM and saw this posting so am replying here to share with others too.
When I purchased my lenses I took the C200 to a local camera store and tried several lenses and ended up purchasing the Sigma 24-105 f/4 Art and the Canon 70-200 f/4 IS. I'd recommend this route if you have a nearby store that is well stocked, I use George's Camera here in San Diego and they are very professional and helpful.
In my experience in the store when comparing to the Canon equivalent, the Sigma acquired focusing faster than the Canon and face tracking worked much better too. The IS seemed smooth enough to me although the Canon might be slightly smoother, the Sigma OS is very quiet compared to the Canon's IS. I have yet to try the various AF response and speed settings so can not offer opinion in that regard as it works well for me with stock settings, although recently I read that Sigma had updated the firmware in the lens to be more compatible with the C series (here it only mentions C300MKII, there was a better article elsewhere but I'm unable to locate the reference):
The zoom and focus rings are very well dampened allowing me to get somewhat smooth zooms and I think with practice I can improve on this.
And the build quality is exceptional with edge to edge sharpness. Here's a comparison between the Canon and Sigma, although this is a few years old and may not feature the latest version of the Canon the results are similar to what I experienced when testing in the camera store with the latest version:
Sigma vs. Canon! Sigma 24-105mm f/4 'Art' lens review and comparison
Finally my main purpose at this time is for interviews, corporate work, conferences, etc., and the lenses I bought should be more than enough to cover my current needs.
Thanks Ron for the for the excellent information and link. I have seen the youtube video, which is where I saw the slight stepping or jerkyness of the the Sigma IS. Another person has confirmed the Sigma IS is good, but not quiet as smooth as the new Canon 24-105 II. Somebody else also confirmed the AF response and speed settings do work with the Sigma 24-105. Sounds likes the Sigma firmware update will be worth the price of the USB dock.
Drew, you may not need the USB dock if the lens is new, meaning not old stock that has been sitting on the shelf for awhile, but if you buy it used you might want the dock to insure the most current firmware. I'll probably buy one eventually once I have a few more Sigma lenses, I like what I've seen of them so far and will eventually get the 30mm f/1.4 Art for low light applications.
I own a C200 with the Sigma 18-35, 50-100 and 24-105. All work perfectly and you do get both focus adjustments ie Speed and Response on the 24-105. The only issue is you can hear the AF with the on board mic and Shotgun mic attachment depending on your sound levels and mic.