Comment I would stick with Plex (Score 1) 420
I have only ever had a few MKV's that made Plex hiccup but that was down to corrupt files. I do get a lot of files it misses but thats purely down to misnaming them.
If you are happy to put yet another overpowered machine under your TV then XBMC is great but from a convenience point of view, get used to naming your files and use plex. I have a small 6Tb centos box feeding 2 Roku's, 3 PC's 2 Android devices, my macbook (even when working away with no additional configuration) and 5 friends with Rokus and Samsung smart TV's.
It can take a little hands on to keep the Library in check but a good bulk renamer and some common sense soon has it all in order.
I think that XBMC is great for the same people who would rather use an old PC with a few network cards in them to roll their own router/firewall over using a small power efficient router with a flashed firmware. Its good but not entirely scalable throughout the house without a lot of MySQL and config tweaking.
Stick with Plex is my vote.