I’d go with the A4 first and use it alongside the MD for a while. Get to know some of the advanced sequencer and file structure features that will be familiar once you get an AR. You can get some really lush pads and drones out of the A4 as well as some huge analog percussion sounds to compliment what the md can do. You can also capture some polyphonic sounds with the UW side of things and keep more analog tracks free for other duties.