Babies don't get fleas. Puppies can and do. Puppies don't let you sleep and tea and cookies at 3AM don't help you out either. You can loose weight running an older dog, not a puppy. Babies grow out of their clothes, fast.
Puppies outgrow their collars.
Babies chew up the furniture. Pupies also chew up their leashes. Your slippers and shoes. Come to think of it, that is an advantage. Babies sleep in clean, soft, warm beds.