I’m going to answer some of your questions from much earlier, as this is quickly becoming an “I like your old stuff better than your new stuff” thread like the ‘Reissue’ one 
Mod wheel/aftertouch/pitch bend: AR might respond to mod wheel and pitch bend, but there is no way to map parameters for these on the AR. It does respond to aftertouch and velocity, and the pads are both velocity sensitive and have aftertouch. People hate on the pads and they’re nothing like MCP/Machine pads, but they’re fine. I’ve had my Mk1 for 6 years and either they’ve softened or I’ve got used to them.
Expressiveness: AR excels here, and is the one place nothing much else comes close. People love to go on about the 16 assignable lfo’s of the MD, but check this - each pad on the AR has 4 assignable destinations for velocity and 4 for aftertouch. These can be positive or negative values. Then you have the 12 performance macros. And then 12 scenes. AR is modulation heaven - but it’s meant to be played rather than ‘programmed’ modulation. It’s sounds like this is what you want when you say ‘expressive’.
Melodic use: it’s reasonable - samples and some machines do well. I keep pointing people to this thread when they want to know if the Rytm can be used for melodic material - RYTM ambient machine
Also have a listen to @George_Michael aka Glo-Phase’s iconic AR sets. Lots of melodic pieces, many early ones solo on the AR - https://www.elektronauts.com/t/glo-phase-ar-sets
Also @KOTARE made some mind blowing stuff with the Rytm, different kind of direction - https://www.elektronauts.com/t/textural-rytm
Having said that, MD with the MCL is pretty damn good for melodic stuff now that you can just play a keyboard right into it.
They do cover very different sonic territory. AR feels like being wrapped up in a woollen blanket. The blanket can be scratchy though, as the machine is terribly frustrating at times like all the Elektron’s.
MD can be warm too, but I know what you mean by ‘warming up digital’ - it’s warm like a cup of tea when your hiking on a cloudy day in winter. Fleeting!
Ultimately it’s going to be a preference. People here are going to tell you one is better than another, but they’re just wrong. Only you can decide that. They’re both excellent, and I wouldn’t part with either.