Undercoating itself is a decent sound deadener, hence the reason a lot of our cars came with some form of it on the insides of the quarters, inside the doors, etc.
I've seen guys have success with truck bedliner material on the undersides.
You need to ask yourself about somethings first.
Undercoating will clean up rather easily with just a spray of water, getting the dust and grime off. The truck bedliner has a tendency to chalk because if it's waterborn nature, making clean up a little harder.
What's the cost? Undercoating can be shot out of a schutz gun and a whole car can take a gallon or so. Some of the spray on bedliners, to get thick enough to be truly effective need a lot more sprayed on and can require a special gun, depending on manufacturer.
How will you be driving the car? Gonna see a lot of dirt roads? Any salt on 'em? The spray on bedliner will probably help with the impact of road debris on the metal, but undercoating is a specifically made rust proofing.
Gonna ask the shops to reach in and squirt the frame rails through the control holes? Undercoating will probably help with that in that it won't have to go in so thick to be effective that it won't allow drain back of water. This you might want to go with a combination of the two once you've made a decision.
