Will I need a root canal or a filling?

It depends on how deep the cavity is. If the decay has reached the pulp (the blood vessels and nerves inside the tooth) then a root canal will be needed. However, you say it's a "slight toothache" so there is a possibility that the decay has just ALMOST reached the pulp but can still be saved by a simple filling, as decay of the pulp is actually quite painful. And yes, a root canal can be painful at first, however local anesthesia is always administered and after that it'll just be a pretty long boring hour or two for the patient, in which the dentist picks around in their tooth.