I love my 600D - I use both my HFS10 and the 600D, but the 600D imho is so much better in low light situations. By now I am really getting the hang of manual focus. I've also learned to not be afraid of using iso 800 / 1600- all to get that better exposure and be able to narrow the aperture. Downconversion to SD kills most of the noise anyway - my nicest shots had videonoise in there.
Anyway: Just make sure you pick the right tool for the job. If you have time to choose your frame, set exposure and focus- take the DSLR. If you have a videocamera backing you up: take a DSLR. For all else: a videocamera. But so far I've found the DSLR makes for the prettiest shots and best low light performance.
I will definately upgrade if the reviews are good on image quality / moiré and autofocus. A bonus if the recording time has also been increased. I already have a nice lens the 600D batterygrip, four batteries and lots of Sandisk SDHC cards anyway. Though I'm also thinking of upgrading the HFS10 to a HFG10, which incidentally fits the HFS10 batteries and accessoiries I have. Prolly will wait for a price drop though.