Oh, I just read that it’s your first piece of hardware. In that case, I’d recommend going with the AR. It’s a little pricey, but you get a little bit of everything. Analog synthesis, sample playback/manipulation, a bit of granular, and hella good drums. AND you can OB the tracks directly to your DAW.
If you’ve already got the A4, go with the MD. Otherwise go with the AR
Edit: if you’ve got an iPad you can get the Strom app and near instantly transfer the samples from your iPad to your AR.