The main reason I got the MD instead of the UD is that i wanted truly wireless connection.
In the past I was connecting the keyboard to the iPad via a cable. With the MD-BT01 I have accomplished a truly wireless connection between the Genos and the Ipad Pro via bluetooth.
The MD connects to the keyboard MIDI ports and it is powered by the keyboard (via the MIDI ports), it does not require additional power. It is completely wireless. Very unobtrusive. No latency.
The UD connects to a USB port in the keyboard. It does not get power from the keyboard, you need to power it with an external power adapter.
There might be other differences but for me these were the important ones:
1. with the MD i am not using valuable USB keyboard connections.
2. I wanted to replace the cable connection and go wireless. With the UD, i would only be replacing one wire by another wire plus an external power adapter.
3. if you do gigs outside your home, you want to minimize the items that you need to carry with you.
4. In my case the MD always stays plugged in the back of the keyboard. I do not need to worry about it.
In fact there is another brand name that is even smaller and good reviews (I forgot the name). But i wanted to go with Yamaha brand name thinking that compatibility should be no issue with Yamaha/Yamaha all the way.
Both MD and UD cost the same about $50.
Other people might have a different situation and go for the UD . . .instead of the MD. For example if they want to use the MIDI ports for something else.
Hope this helps.