I'm with Soozll, especially if the recipe is an old one. Dutched cocoa is a relatively new pantry item, and even today, not all grocery stores keep it in stock.

Most cookbooks specify Dutched Cocoa when that is the one that should be used. I would go with natural cocoa.

What are you baking? As stated above, if the recipe calls for things like citrus juice, buttermilk, sour cream, yogurt, or the like, these things will provide the acidity needed to generate a reaction with the baking soda for leavening. The baking powder will result in some leavening without the presence of acid.
So, since you have both, it doesn't really matter which type you use.
In general, cocoa powder is acidic which is why it reacts with baking soda. When unaltered it also provides a deep chocolaty flavor.
The dutch processed cocoa is made so that its acid properties are neutralized, thus it will not react with baking soda. The only time you must use dutch processed ("alkalized") cocoa is in recipes that call for baking powder, unless there are sufficient quantities of some acid also call for.
DP cocoa also tends to have a milder, subtle chocolate flavor.

That said, I'd probably go dutch processed in this case.

What else is in the recipe? Is the soda intended to neutralize the cocoa (ph 5 if not dutched) or some other acid. If there is no other acid, I'd go with regular. Per Harold McGee "Some recipes rely on acidic natural cocoa to react with baking soda and generate leavening carbon dioxide. If the same recipe is made with an alkalized cocoa, the reaction won't take place, no carbon dioxide will be generated, and the taste will be alkaline and soapy." Now, the baking powder will generate leavening even if the soda reaction doesn't happen, but I'd be worried about a potentially bad tasting outcome.