Generally, the spray-on stuff will be a lot lighter, and will be far easier to apply in difficult area (i.e., not perfectly flat). I'm not talking about generic undercoating, but rather purpose-engineered sound deadening materials. I got a case of spray-on stuff from my friend when he worked at Tweeter (got it at cost), and even then, it's very expensive. But, it seems to do the job, and it does weigh far less. For instance, the guy I got my ITB Ascona chassis from had applied a bunch of extra sound deadening under the hood, the firewall, the doors, the floor, the rear deck area, the trunk lid....I removed over 140 lbs of the conventional type (Dynamat) sound deadening material. Yeesh. My Dad's GT is getting the lighter-weight stuff however, even though the engine will be powerful enough to make up for the extra weight.
Bob