You don’t absolutely have to have both, but I would recommend it if you can. If I could only choose one, it would be a Dutch oven. They’re more versatile and in a pinch they can do just about anything a stock pot can. But, I would hate to have to pull out my heavy dutch oven every time I want to boil water for spaghetti. Not to mention, a good enameled cast iron Dutch oven is going to cost significantly more for a much smaller size.