Agree with beetle, most people use to refer to a 6-city empire as wide, if you founded the cities yourself, or at best it would be transitional between tall and wide. Also by stronger, do you mean which one wins the faster? I think it's fairly well established that a 4-city Tradition strategy will bring you the fastest (science) win, while if you go for culture victory, I think wide generally serves you better (at least from my own experience) because you can hoard more artifacts, more religious buildings (if you have Sacred Sites), more Landmarks etc. within the borders of a wide empire.