I bought a few cans of both Rustoleum professional rubberized undercoating, and a few cans of Dupli-Color professional rubberized undercoating, and sprayed them on some cardboard so that I could compare the two, and I thought you guys might benefit from the findings.
Both advertised the same benefits, that they were sound deadening, flexible, paintable, etc.
I found that the dupli-color went on thicker, and you could get much more build with it than the Rustoleum. The dupli-color also ended up being much more flexible. I cut a 1 inch square out of the middle of both test sprays, and the rustoleum actually flaked apart and was not very flexible. The Dupli-color was very rubbery, and seemed to hold up better.
Unfortunately I did this after spraying the rustoleum in a wheel well... so I will be removing it to re-apply with Dupli-color.
From what I've heard, dupli-color is comparable to 3m in its thickness and application.
I don't have any comparison with longevity, of course... does anyone have any info on how either have held up over time?