The MU2 is a streamer DAC combo. It also has built in Roon Core and upscaling.
MU1 is streamer only. It has the Roon core and upscaling, but no DAC.
Both have optional onboard storage for your library.
I use the MU1 and an external DAC. I do prefer seperates. I don't want my DAC attached to my streamer in case I want to change one or the other. that said, there can be some advantages to designing the DAC/Streamer to maximize snyergy, avoid the need for a second set of support equipment (poer supplies, cables, etc).
I've heard nothing but good about the MU2. In fact I bought my MU1 from a guy who was one of the first buyers of the MU2 over a year ago.
Jerry