You might have tried a savory recipe that called for tamari and wondered if you could use those leftover soy sauce packets from your last takeout sushi night. The short answer is yes. Both soy sauce and tamari are are byproducts of fermented soybeans, making them similar in taste and color; when cooking, the two are pretty much interchangeable unless you are gluten-free since tamari is most often made without wheat. If you're living the g-free life you still need to read labels because sometimes wheat will still be included — I'm looking at you Kikkoman.
I wondered, gluten-free aside, was there a reason to choose one over the other? I checked in with the registered dietitians of C&J Nutrition, and they don't have a strong recommendation to choose one over the other nutrition-wise but advise their clients opt for "lower sodium version of both soy and tamari" — since one tablespoon of either contains close to 1,000 mg of sodium. For reference, the FDA daily value recommendation for sodium caps out at 2,300 mg. But if you prefer the flavor of the full-sodium versions, you can keep your sodium in check by adding a half teaspoon at a time and adjusting the flavor from there.
Regardless if you go with tamari or soy sauce, choose a brand with fewer ingredients. They should mainly be: soybeans, water, wheat (not found in most tamari), and salt — some also contain alcohol as a preservative.