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XBMC - Use a dedicated HTPC box and run the linux/windows version of XBMC.
PLEX - Use a Mac Mini.

If you want the option to run XBMC and PLEX on the same box then use a Mac Mini. If you only want XBMC then a PC based box is much cheaper.

The ATV2 will run XBMC and a stripped down PLEX client but you asked for the best hardware, and while it's the cheapest option, it's not the best hardware.
 
XBMC - Use a dedicated HTPC box and run the linux/windows version of XBMC.
PLEX - Use a Mac Mini.

If you want the option to run XBMC and PLEX on the same box then use a Mac Mini. If you only want XBMC then a PC based box is much cheaper.

The ATV2 will run XBMC and a stripped down PLEX client but you asked for the best hardware, and while it's the cheapest option, it's not the best hardware.

I am quite interested for Mac Mini and to have also Win 7 Pro installed on it as a backup if Mac Os update would lock the OS because their business is Itunes. Maybe I am paranoid but this has been the plan but would you recommend Mac Mini standard or Mac Mini Server?
 
Custom made HTPC (maybe AMD Llano or AMD E-350 based) would be the best hardware for it, you can build it to even support 3D bluray playback if you want to ( and have a 3D tv or plan to get one soon).
 
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