MIDI and MusicXML are currently the only two options for transferring data from Dorico to Cubase. MIDI will include the keyswitches so that if you load the same plugin then it trigger the same articulations. However it won’t transfer the actual expression map. MusicXML won’t include any keyswitch data. If your workflow is to transfer to Cubase for general polishing up and mixing then MIDI is the best option. MusicXML is only worthwhile if you are transferring to another scoring package.
Dorico’s expression map format isn’t so much a superset of Cubase’s. It has a number of similarities, but the key thing is that it represents the capabilities of a plugin or sample library in terms of what Dorico needs for score playback. So one difference is that there’s more semantic structure about a wider range of specific playing techniques and the relationships to each other. We intend to extend what can be represented in the expression map in terms of how Dorico maps score items such as vibrato indications into controller changes. Dorico will try to import a Cubase expression map, though it doesn’t always map on to something that makes sense in Dorico’s model (we’ve made a number of improvements in this import for 3.5). It’s not possible to export a Dorico expression map to Cubase though.