Different digital methods could make the difference.
Do it over using same methods of transfer.
It’s hard to compare with all the permutations we have for connecting our streamers and DACs. Finding the most synergistic digital path is key for me when testing, not necessarily comparing the same connections. The Lumin U1 Mini has 5 ways to output digital; the Node 2i has 2.
I’ve read many forum posts that say the Lumin U1 Mini’s AES/EBU output may be the best sounding. I couldn’t test that; I don’t have a DAC with an XLR input for AES.
I found the best synergy, and subsequent sound quality, for the U1 Mini to my RME ADI-2 DAC was USB. A DAC with a lesser USB implementation may do better with S/PDIF or AES.
The best sound quality out of the Node 2i for my setup was S/PDIF-Coax to the ADI-2 DAC.
Sound quality between the two connected this way, subjectively: I like the U1 Mini better than the Node 2i, but it isn’t a huge difference. They are both very good.