First of all, there's no blanket answer to your question - it very much depends on how the formats are implemented (I've seen terrific and awful examples of both).

Generally speaking, though, AVCHD format is limited to 720p video and ~DVD video bitrates (because it's designed to play from DVD media), whereas the MKV container can accommodate pretty much anything you like in terms of video and audio streams.

In this instance Harmy has used AVCHD for the first release, but intends to follow it shortly with a high-bitrate MKV; this will offer higher quality picture and audio (albeit still at 720p), which will be playable either on an appropriate media player or by burning it to Blu-ray disc.