I also feel like if someone in your party passes a perception check and spots that a gargoyle head is a fire spitter, you should not need to roll a perception check for each individual gargoyle head. That's a DC 5 intelligence check at best. Once you have spotted one of them, it's easy enough to spot all the others as well.
Raise the DC check to spot any of them, sure, but once you spot that, reveal all the gargoyle heads as traps.
The floor vents are different, because they're concealed in the floor, not obvious on the wall, and they could potentially be in all sorts of different places.
