First of all, let me tell you all that both the devices come with their very own and definitive functions. However, they are used for interchangeable tasks. You can always use one instead of the other, but that may give you some inconsistent results.
I feel the food processor is way more effective if you compare it to a hand blender. A blender works well for blending and kneading, but this is not as effective as the processor. Using a food processor will allow you to process a whole lot of ingredients.