Pivos XIOS DS and the linux/xbmc firmware is the closest in the long run, the firmware is still beta but it has the guy who made the ATV2 and Android versions of XBMC working on it so it is steadily improving.
Also eagle eyed users spotted M6 appearing in the Pivos code, this is the 8726-M6 chipset a more powerful dual core chip then the M3 that powers the XIOS so you might want to wait for that. You can buy generics based off this platform already but they are simply taking what Pivos released and hoping for the best that is runs on theirs.
If you want something more stable, the Raspberry Pi is likely the next option as development is further along, but the R-Pi is a bit of DIY system so your going to need a remote, the MPEG-2, VC-1 licenses from R-Pi store and maybe a case for it too. Use Raspbmc on an sdcard to get it all going.
Personally I don't think much of Openelec, if you still have the R-Pi I would give Raspbmc a go which has hit version 1.0.
Pivos don't have an M6 of their own yet but they are clearly laying the ground work for one with it appearing in their software. A lot of the generic 8726-M6 boxes are basically guinea pigs, sorting out bugs and pin pointing issues so they aren't rock solid stable. This thread on the XBMC forums is discussing the generic 8726-M6 devices if you want to know more.
Basically the Pivos version with it's linux build will be the most capable and stable version.