The first one.
Tomato sauce is standard on a pizza, I wouldn't call it an additional topping.
"The additional toppings on this pizza are pepperoni and mushrooms." works, but I like it much less than the first sentence.
If the tomato sauce, pepperoni and mushrooms are already on the pizza, I would call them toppings. I would say 'the following additional toppings are available: ham, mozzarella cheese, chopped tomatoes, anchovies...' i.e. pizza places often have standard pizzas with a number of toppings on, but they also offer additional toppings, which people can pay extra to add to their pizza and 'customize' it.