The wizard has a high damage per second and a large damage radius. On the other hand, the witch with lower damage per second has the ability to spawn skeletons.
The wizard deals with air troops more effectively since the witch's skeletons won't be able to deal damage to air. Futhermore, the wizard kills swarm troops faster because it has a higher damage per second and a larger damage radius. It takes only a shot to kill a minion while a witch takes three shots to kill it. The wizard would be a better counter to spawner decks with it's longer range and higher area damage.
The witch with lower statistics isn't worse than the wizard since it has the ability to spawn skeletons. The witch can't kill swarms that fast but can kill ground tanks(eg. Prince, Giant) more efficiently since the skeletons could distract and add more damage to it. It can also work more effectively then a wizard behind a tank when attacking a tower(the spawn skeletons plus the witch's skeletons is higher than the damage of a wizard).