What Chuck said: get the lowest density piece of top wood you can and leave it thick enough to get the requisite stiffness. With that plastic back you're going to need all the help you can get, so I'd suggest you look for a decent piece of Western Red cedar. It's usually lower in density than most of the spruces, although there's a lot of overlap between all the softwoods. Also, WRC usually has much lower damping than spruce.
Redwood is quite variable, and, like the little girl in the nursery rhyme,: "When she was good she was very, very good, and when she was bad she was horrid". I've gotten redwood, like the well-known LS log, that combined the stiffness and density of good Red spruce with damping that's as low as that of Brazilian rosewood. I've also seen redwood that had stiffness and damping numbers that were not much better than cardboard. The small experience I've had with 'sinker' wood suggests that there's no magic to it; if the wood was god when they sunk it, it will probably still be pretty good. As always, you need to look at the piece of wood, not the species or the sales hype.
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