These tomato-based sauces are similar in flavor, but there are a couple unique differences. Both have tomatoes, olive oil, and garlic as the base. However, marinara sauce uses onions which gives it a sweeter flavor. It also doesn't require red pepper flakes, so it's not a spicy sauce. Arrabiata is a less sweet sauce focusing on spicy flavor from the red pepper flakes. It doesn't use onions but is heavy on garlic and crushed red pepper flakes.