Basil in the traditional "green" ingredient in pesto. Basil is also one of the three ingredients in a traditional caprese salad. So, if you want to lean traditional, use basil pesto in this sandwich. But here's a secret. Kale pesto is amazing. We make kale pesto all the time and use it with ravioli, as a spread for toast, in sandwiches like this one. We make it all the time because it's delicious, but also because it's a lot cheaper to make kale pesto than basil pesto. I grow both basil and kale in our garden, so it's not like we're running out to the store to buy either one, but it takes a lot more time and effort to grow basil than kale. So I like to keep the basil for recipes that need less and use kale in its place for pesto.