My gut feeling tells me the video quality with Realtek 1185 and 1186 is similar, and the latter only comes with additional features (3D among others) and a faster CPU. The reason I've come up with this supposition(without hands-on experience) is I've gone through the comparison chart of Sigma chips as below(thanks to wongkmc for providing the link).

If you look at the video processing, from the 8630 chip from early 2008 to the most recent 8670 chip(yes, the 8655 chip in the WDTV Live is already obsolete), they all share the same specs in motion adaptive deinterlacing, programmable scaling, chromaticity correction... etc. Only the upcoming 8910 chip will drop the motion adaptive deinterlacing and incorporate the new integrated VXP processing that is claimed to improve video quality. This information also comes directly from Sigma themselves as shown in their own brochure.
To summarise, I believe all Sigma chipsets(8630, 8640, 8650 and 8670 series) all share similar video quality, and the upcoming 8910 chip that will break away from the pack with its new professional grade VXP processor.
pdf brochure here
SMP8910 brochureLink here
Sigma Designs chipsetsAs for differences in Realtek chipsets, very vague information provided by the company. Realtek only specify on the differences in features between their chips on the website (RTD1055, 1185 and the new 1186), hence my suspicion that video quality is similar across the range.
As for Sigma vs Realtek, in a technical perspective which was demonstrated by anfieldsdude with measurements carried out using advanced tools and gadget, in layman terms the Sigma chip produces more accurate colours, less artifacts and better differentiation at the black-white spectrum. The Realtek is said to produce a brighter and warmer colour, though not as accurate but can be pleasing to some, which is subjective.
If one does not need the additional features of 3D and faster processor of the 900B, then the 600B or 600C will do equally well in terms of video quality me thinks.
Added on January 9, 2012, 1:26 pmJust read the 8910 brochure, and the new VXP video processing incorporated into this chip really sounds promising. The player that has this chip will certainly come with a price..
This post has been edited by ryder_78: Jan 9 2012, 01:33 PM