Well it depends but mostly when it is on higher leagues, I can say that Wizard Tower is better than the Mortar since it doesn't have safe zone and can wipe out giants or any other cluster of meatshields if placed properly
Wizard Towers are more versatile. They do more DPS, attack faster and can target air, but they have a shorter range and are usually more expensive. However, Mortars have more damage per shot, which could make a Heal spell useless in certain cases. Mortars are also cheaper to upgrade, and they have quite the bit of range... and on higher levels the blind spot usually isn't too important when you have multiple Mortars.