The MR1 sounded nice, sure, and the balanced inputs were a positive. The hard drive and powering scheme sucked and the max line level signal was +9dbu. It was so well intentioned, but it was hampered by logistical issues... I think I only kept mine for 6 months.
The D50 may not have as nice of an ADC, but it does have an digital (optical) in, and it resolves the two biggest problems I had with the MR-1. Unless I'm in an extremely controlled environment, I would pick the D50 over the MR-1. In my experience it just works better in the field. If I was doing outboard studio work and wanted to do a final mix A>D in both LPCM and DSD, then I'd reach for an MR-1.