They're all pretty comparable. One thing I look for is the bit rate of the video output (posters putting screen caps helps out too). There have been a few cases where MKVs with slightly lower bit rates have looked better due to better compression from more experienced rippers, but overall the bit rate level is the best way to judge.
DarkGryphon2048: while you can create AVCHDs out of the MKVs (with programs like TSmuxer), you usually have to start off with the MKVs since that is the original source material (at least if you get your stuff from tehparadox). I just wound up getting one of these:
I stuck in a 2 TB hard drive and so far this plays everything I've thrown at it: MKVS, FLAAC, MP3 and 4, etc. It has a GUI, so when you plug it into your HDTVs HDMI connection, you can access all of your programs, create file folders and delete extraneous files. Bottom line: I don't have to worry about converting MKVs anymore so they will play on my PS3 (as it outputs in 1080p as well).
I dunno I prefer having stuff on physical media compared to having to buy even more hardware for my room. Except I'm considering purchasing the Sherwood 5004 multi-region Blu-ray player which is a clone of the Momitsu.
Yes I have quite the Blu-ray import wishlist.