The XIOS would be the way to go but it is still very early days, the linux firmware has really only just gone through alpha strange and the first proper beta 1 is due to be released soon. In other words expect issues but they are hunting them down and nailing them.
The XIOS supports MPEG-2, VC-1, MPEG-4, and H.264 via hardware however it seems Pivos is still feeling it's way through the hardware so I don't think anyone can say with absolute certainty on what it handles as it is still maturing, it's played most things I've thrown at at it and does handle some 1:1 BD rips done via MakeMKV. The linux firmware is on par with the AppleTV2 version of XBMC performance wise more or less. 3D support is unusual at first it didn't play H.264 MVC videos but now it does, however the one demo clip I had was low enough bitrate it could have been the CPU decoding it but a few firmwares back it refused to play the same file at all.
The best thing to do is head over to the Pivos forums
and post a sample of the videos your using and ask anyone to test them, that way you know for sure at least if it can play such files.