Maybe it's just me (or maybe I've invested too much money into headphones), but I actually liked the Opus sound better at 6 kbps. The MLow samples had these... harsh and unnatural artifacts, whereas the Opus sound (though sounding like it came from a tin-can-and-string telephone and lacked all top-end) at least was 'smooth'. But I'm pretty sure that's because they are demonstrating here the very edge of what their codec can do, at higher bitrates the choice would probably be a lot clearer.
