Chuck,
It's kinda like "Ford vs Chevy". There are advantages and disadvantages to each.
Cork probably rides the water better, especially big, rough water. A solid cork bird will absorb shot and still float, where a hollowed bird hit hard enough can take on water, crack, etc. Cork is much easier to carve; you don't have any grain to deal with, and power tools will absolutely grind through it quickly.
Wood, especially tupelo, cedar, etc. can be carved in almost the time of cork once you get the hang of it. You can hollow a wooden bird and get it light; if you had the right tupelo or white cedar, for example, you could really feel the difference. This is an advantage if you have a weight limitation, or are hunting areas with shallow water and light winds...lighter decoys move easier.
What you carve really depends on where you are, what species you gun, and what you want out of the decoy. Does it see 50 days of a 60-day season, or 3? Do you gun flooded corn/rice/marsh, flooded timber, Lake Michigan, or either coast? Are you going to use individual pocket decoy bags, or throw 'em in a gunny sack or the floor of the boat?
Materials cost may be an influence, too. If you can get basswood, cedar, or tupelo for $1 a board foot, kiln-dried, carve wood. If it's working out to $50 per bird in just the wood, think about cork to ease up on your pocketbook. I'm exaggerating on both ends, but you get the idea.
Try the tan cork. Chances are that it will be cheaper than the basswood blocks you're buying, and you can learn without fighting the grain. Think of them as "learning blocks" too - you wanna try something new, try it out in cork first...