I think the main things to think about in regards to blade width is surface area and force. Think about the force you want to apply and think about the force the water will apply on you. A narrower blade will be easier to control and will get jerked around less in turbulent water. With a narrower blade you will not be able to move as much water i.e you will not be able to move your boat as much with one stroke. If you like to muscle your boat around or like to take long deep pulls (that statement could be used in other forums as well, but let's stick to the topic at hand), you may prefer a wider blade. If you want to feel like you have more control and like to take shorter quicker strokes (again, let's stay on topic here), then you may prefer a narrower blade.
You probably should consider the load you are hauling as well. To move a "big" boat you may find the wider blades better.