Hi Dave,
If you're open to a suggestion, I'd go to the Lumin website and compare the specs for both. As for sound quality, my thought is that given that the room acoustics are a big factor with respect to how a system sounds, IMHO, the only way to unequivocally determine would one would sound better would be to compare both units in your room with your system components. That would be the most accurate way to do a controlled experiment, from a scientific perspective, as it were. .
If I were to hypothesize, it would be that the U2 Mini would have better performance and audio quality since it is a newer model with newer specs, features, and functions. Cheers.