For potting I think you want to go by a couple general rules of thumb:
- how wet does this plant like to be?
- how wet is my climate?
- how much do I water?
The relationship of those three will influence how you should pot. If for example you have a plant that doesn't want to dry out, but you live in a damp climate and tend to overwater, you will want to pot in something that drains well, regardless of the plant wanting more water. A miltassia will be in the medium range in terms of water requirements, so it sort of depends most on how you water. If you water a lot, choose something that drains well, if you tend to forget to water, then you want more moisture retention.