A chef will look for both variety and ripeness in a tomato. There exist a wide range of flavor profiles among the different varieties. We use a yellow Russian tomato called Azoychka for our Sicilian tuna-stuffed tomato because the more subtle acidity complements and does not battle with the acidity of the pepperoncini and capers used in the mix.
Any chef will prefer using San Marzano tomatoes if making a red sauce for pasta. But the most important thing a chef will consider is ripeness; if they can procure tomatoes ripened on the vine in a local garden, they will choose them 100 percent of the time. Most tomatoes—even organic heirlooms you find in quality specialty markets—will be picked early and will never develop the complexity of a vine-ripened tomato but will benefit from being allowed to ripen at room temperature for a day or two.