Bro, there is a lot of difference between the two, underlay reduces road noise where as dynamat does that to a MUCH greater extent, with dynamat comes excessive weight as well, which reduces rattles a lot, it is also heat resistant, therefore it helps in keeping the car cool in summers.
Since you add weight to your doors, you're reducing the resonating frequency of the metal, thus rattles are reduced a lot. And also it absorbs vague sound frequencies which might cause phasing issues.
Yes it iis quite expensive, a single door kit would set you back around 10-11k
But it is worth it.
