Are enameled cast iron pans or stainless steel pans better for cooking with acids like tomatoes? I am wondering if I can use all cast iron/enameled cast iron pans, or if I need a stainless steel pan also.
Enameled cast iron usually means porcelain enamel, a type of glass. It's resistant to both acidic and alkaline foods. It is fine to cook and even store tomato sauces, etc. in enameled cast iron.
Stainless steel is also resistant to acidic foods, but not as much as porcelain enamel. Storing tomato sauce in it may eventually discolor the pot.
Both stainless and enameled iron are perfectly good options for cooking with acids. Whether you need a stainless pan depends on what cooking you're doing. In brief, cast iron has very slow heat response (e.g., you raise or lower the burner, it takes a while for the pan to cool or heat); stainless is much faster. That's a drawback too, iron gives a very nice steady heat, perfect for braising.
Stainless is also almost always lighter. I have a 12 qt stainless stock pot, its several pounds. My enameled iron dutch oven, at less than half the capacity, is closer to 15 lbs.