I agree with Lorene, a retractable leash is good for certain situations but I think it's a good idea to have both. I've also heard horror stories on dogs pulling too hard and breaking the retractable leashes and getting cut or sometimes slicing the owners hands.
I think on a walks all you need is a normal nylon leash and when you are in an area like a park where they are not allowed to be offleash then use the retractable leash so they can roam.