Grain’s kind of beside the point. Grain is supposed to be there, it’s the film equivalent of pixels. The AVCHD will likely have less grain due to a higher degree of compression, while the MKV will be less compressed and thus more able to preserve fine detail, such as film grain.
The image I linked showcases the difference between minimal and heavy compression. The MKV is less compressed than the AVCHD, so if you notice any difference between the two, it will be similar to that “Avatar” comparison.
Having said that, the AVCHD versions are very carefully compressed, so they still look quite good. But if you’ve got a larger screen or sit fairly close to it, the compression artifacts will become more apparent and the MKV will be a worthwhile upgrade.