polymer isn't always lighter. I like the look of wood but I'd have to say that a synthetic material has the tactical advantage. Every tree is different. This means that wood stocks will vary in weight and density with each tree. Not consistent, it's the same with guitars, even identical model guitars will sound different and can weigh considerably different weights, for otherwise being identical. Polymer will be more stable in different weather conditions and should handle the barrel heating more tolerably.
On the other hand, I have heard accounts of certain wood stocked rifles being consistently more accurate than the same model with a polymer stock. This obviously isn't true for all rifles.