Primary concern is sound isolation in the basement. Ideally I don't want any sound travelling through the rest of the house.
Putting on the upper and side and not the lower is purely to allow more ceiling height, as the drywall would not be able to sit as close up to the returns.
I've heard the purpose of putting it on the returns is just to have something heavy on the returns so they don't have as much of that tin noise (i.e. when I just tap the return now you can hear it right up through the vents, but by putting something dampening like that on them it will deaden the sound through them).
Noise from airflow should be an issue as I'm stuffing safe&sound in the ceiling joists and around the s-shaped pipes. Could be a concern for heat flow though to the rest of the house.
