Basically, if you don’t notice any difference between the “original” and “compressed” sides of this picture, then the AVCHD is probably plenty high quality for you. If you can tell the difference, then you’ll probably notice a quality bump with the MKV.
You also won’t have a comparison running through the whole movie. I can tell the difference, but I doubt I would while actually watching the movie, I only notice because the uncompressed image is right next to it.