I may be stepping on a hornets' nest here, but my opinion is that both have their time and place. In general, most rotisserie cooking is done over indirect heat, to prevent burning the surface, and that's the way I roast chickens, prime rib, and pork rib on a spit. I started my professional career cooking ducks on a rotisseries at a French bistro in San Francisco. It was called Roti. Anyone remember it? Anyway, we cooked ducks on a rotisserie over direct heat, but the there was a a yard a space between the fire and ducks, so no worries with flare-ups. I would not do that on a kettle. But I would roast a pineapple on a kettle over direct low heat. I'm not sure if my experience is what you might call "insight." The smartest thing I can say is, do whatever works for you and the equipment you have. Einstein said that, but not really....
