They're the terrible two. The eternal rivals, the Frazier and Ali, of British art. Or perhaps Picasso and Matisse would be a better analogy. For while everyone knows Ali is indubitably superior to Frazier, as with the two great continental modern masters, Turner and Constable are each supreme in their own way, and they are such different ways, it's impossible to quite say which is better than the other. Except that people, even experts - particularly experts - deep down do inevitably feel more passionately about the one than the other. And which of them you prefer says a great deal about you, what you feel about art and British art in particular. Not only that, the prevailing view on the great...