HVAC vents - does wall or ceiling mount matter?
I think part of this depends on where you live, and whether this is primarily a cooling or a heating issue. I'll assume we are talking the high velocity vents, which are easy enough to move around. (A check of your profile says San Jose, so I also assume this is a cooling issue.)

Wall mounted vents used for cooling will push cool air across the ceiling, mixing with the warmer air up there. The cool air will now gradually settle down, because cool air is denser than warm air. What you won't feel so much is moving air.

Ceiling mounted vents, because they push the air DOWN, not horizontally, will arguably create more drafts in the volume that you actually inhabit. This is something my wife would not want. So if you do put them in the ceiling, place them where you will not feel the drafts.