Believe it or not, for flooring, concrete is the best. Wood holds germs, absorbs urine, it splinters, and gives a place for fleas and ticks to hide. If the hardness of concrete is your concern, there are rubber mats made just for kennels. They are not solid so they allow circulation and they are much softer than concrete.
Dogs, themselves, prefer dirt and grass.

Actually, they prefer to be with their "pack", but labs can be a handful. I know, I have 4. They are diggers, so that would be another reason to use concrete. They will still dig, bite and chew wood and dig through gravel or dirt to get out of the kennel. They are dogs that love humans and love to have someplace to run. They need
lots of exercise.