This is an update on the current scenario since my previous post in March. The HD media player world is still dominated by chipsets from Sigma and Realtek. Sigma came out with the first HD media player chipset in 2008 while the cheaper Realtek chipset came out a year later and majority of the HD media players are now using chipsets from these two manufacturers.
Here's a partial list of popular media player brands using the two chipsets to give potential buyers some idea. Realtek has more manufacturers but mostly in the medium to low end market while Sigma has more players in the high end market apart from the popular WDTV Live. High end players will have more functionalities and may even have a blu-ray player built-in.
Sigma - WDTV Live, Popcorn Hour, Egreat, Dune Smart, HDX BD1
Realtek - ACRyan PlayOn!, Asus O!Play, Xtreamer, Patriot Box Office, Seagate Freeagent, Egreat, Yangxi HDPro+, Noontek, Eaget, Hornettek Phantom, DVico
Below are some items for comparison.
Video format support
Realtek - all common file formats.
Sigma - same as Realtek except it cannot support RMVB.
RMVB (RealMedia Variable Bitrate) files are very popular in the distribution of Asian contents such as Japanese Anime and Chinese TV series and movies so if you are going to use this type of files, then you must pick a media player that uses the Realtek and not the Sigma chip. Thus the popular WDTV Live is out in this case as it uses the Sigma chip.
Audio format support
Sigma - all common audio format including HD audio such as Dolby TrueHD and DTS-HD MA.
Realtek - same as Sigma. Earlier versions of the Realtek chipset could not passthrough these HD audio but newer versions now have overcome this problem.
PQ or picture quality
Sigma has the better specs on paper but many prefer what they see from Realtek. There are supporters in both camps so it is very much a personal preference.
The Sigma chipsets generally have higher clockspeed than Realtek chipsets but probably not significant in real world applications.
Please note that the firmware used can also affect the performance of the player. New brands and models are coming out fast and furious and some may even change camps (Egreat seems to have models in both camps) so best to check the specs of a particular model to see what they can offer. The best is to bring along some test videos and physically check that your video can be played if this is possible, before parting with your cash. The iboum website is a good place to learn more and check the specs of most commonly available HD media player.