To be honest, my impression is that the sound quality doesn't differ all that much, and they all sound really great. If I had to point out their characteristics: Apollo's strength lies in its exceptional Universal Audio ecosystem (as they call it) which emulates vintage hardware compressors, offering great bonuses beyond just the hardware emulation. RME Babyface has been the go-to for professional environments for ages, featuring rugged reliability and a transparent sound. Zen Quadro probably has the highest specs on paper, with a slightly brilliant sound profile that I think makes it great for listening, even though Antelope Audio's website is quite hard to use. That's about it. Also, there's quite a bit of compatibility with monitors and other gear, so it might be a good idea to consider your choice based on that compatibility.