I'm using an HDMI cable for both. And I played both demos again and at least in my case the 360 version have crispier sound effects, for example, I notice the difference when I take the spine from enemies.
I don't know then but there really is no difference in the sound quality on my end and I use an HDMI as well. Dunno
The only differences between the two have been stated many times.
PS3: very slight better video quality compared to 360: runs slightly better