Apart from the first model they all have the latest chipset EM8634. This chipset is still very much in developement. Therefore so are the media players to some extent. Tvix is the more mature company and has its own firmware. The others use NMT from Syabas. They were first into the 8634 market and are coping well with the fact that Tvix were more established overall.
The Tvix is double the price of the PCH. You can see where money has been spent over the PCH but do you need the extra it offers? Both are not easy to set up and you will fumble around for a bit. No doubt you will find yourself an active user of eachs forums at first. The PCH has the edge in terms of internet access for picasa,youtube etc.
If you want a polished nice looking easy to use unit and money is not a major factor then perhaps the Tvix. If you want a cheap player for primarily video playback then the PCH/NMT players are the better bet.
There are reviews of each and the Istar at MPC:-
http://www.mpcclub.com/modules.php?name=Reviews
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