I had an md and now the AR. To be honest, the md feels a bit outdated right now. I felt the md had a few sweet spots, but the rest of the sounds were not useable to me or sounding thin. The only useful machines were the tr and fm machines on the md, and those are covered with the AR and sound alot better to me.

The pads from the AR also help to make your sequences more organic.
Also the sequencer and performance features is missing alot of functionality compared to the AR. No usb on the md either. No overbridge in the future. There is over 10 years of innovation between the two and it shows.