I have no experience with spray deadener. I'm sure it's perfect for hard-to-reach areas, but I would recommend removing the panels in question and applying sheets directly to the metal vs drilling a hole and using the spray.
Sound deadening works. My car is quiet from the factory already, but sound deadening the doors so far made my car so **** silent. I sometimes dont even hear my engine idle. lol.
Stay away form spray. Hassle. Go with the mats. Dynamat is good, my car is covered in it. However, Damplifier is just as good and much cheaper. Go with that.