Pretty much anyone you talked to who has used both dynamat and second skin will tell you... second skin is the superior product. I used to be an installer for circuit city, and we sold dynamat. It is a good product, don't get me wrong... but compared to second skin, it's harder to handle, harder to cut, and it's crazy expensive. Second skin does its job just as well as dynamat, but at almost half the price. Something that was funny to me is that my friend who works for sound advice, and with his discount, he can get 12sf of dynamat for $40, or he can buy second skin at full price... $50 for 20sf...
So I say second skin is the ebtter choice because it's a higher quality product for a lower price.