I would think about which has the best weight/cost ratio. Heavier = better sound deadening.
I don't want to be an instigator, but heavier won't necessarily equal better sound deadening. For the most part that's true, but long-story-short, a "softer" metal will deaden better than a "harder" one because less sound will be reflected. Mass doesn't have much to do with it unless you are COMPLETELY shutting up the box, which 1HandClapping isn't planning on doing. In this case, you want the least reflective surface (which, 1HandClapping, probably isn't going to be metal, but heavy foam. Remember MikeC's rule number 1 on enclosures, "hard drive noise leaks wherever there is a gap in the sound barrier." So if you don't use a sound ABSORBING material and you leave the end open, your going to end up with sound amplification from the sound REFLECTING walls.)
If you don't mind waiting, wait until I try out soundproofing lead. I ordered 1/32" thickness which = 2lb/sq.ft... and although it's toxic, it's easier to work with. I'll probably line the lead with Al. foil to minimize skin contact. It'll be inside the case anyway, so I'm not too worried.
I might think twice about this approach if I were you. Unless the enclosure is going to be fully out of the way of airflow (which wouldn't make much sense for an HD enclosure), then you'll be fanning lead particles all over the inside of your case and coating your components with an ever-so-miniscule toxic film. You'll also be blowing said particles out the exhaust of your system, ostensibly into the air that you breath while seated in proximity to the computer. I would strongly advise avoiding lead in any such situation (FYI: While no longer in the game, for several years I worked for a contractor in PA who both built and inspected homes. There are very low thresholds for lead exposure in some states, reflecting the very serious problems that can occur due to even limited exposure.) Just a word to the wise.
And to cap off a very much overly long posting (sorry):
I also found a manufacturer that make thermally conductive elastic. They have both electrically conductive and electrically resistive types.
Who's the manufacturer and will they sell me some!! I'm going to try a vertical RAID suspension in the HD cage of my case, and I'm very concerned about the temp my drives reach. This would be a great asset to keep the drives cool.