Buy a trained or least started dog no matter what kind of dog you get. Otherwise, you have to wait until they grow up to use them. While you are waiting, you may find out they aren't suitable as a hunting companion or have health problems, and you'll just have a pet.

If you will primarily duck hunt, buy a lab. They will work fine as a flushing dog and will be superior for waterfowl and pets.

Be warned though that many labs are hyper which goes back to my point about buying a started or trained dog.