How many productions of "David Copperfield" do we need per year? TNT and Hallmark Entertainment apparently think we need two this year Back in April, we had the three-and-a-half-hour PBS/BBC version, which was a winner - beautifully produced, acted and filmed. This month, we get TNT's four-hour version, which, if you leave out all the commercials, comes in at just over three hours. It, too, is a winner for the same reasons - excellent photography, sure-handed direction and first-class casting. But if I had to choose between the two, I'd have to give last April's version a slight edge: The casting was a shade better. Newcomer Hugh Dancy has the title role in TNT's "David Copperfield," and he's ably assisted by three-time Emmy winner Michael Richards as Mr. Micawber and two-time Oscar winner Sally Field as Betsey Trotwood. Dancy looks so good in the role of the grown-up David that he'd barely have to act at all to be a success. Nevertheless, he doesn't miss a single step. As a result, his almost effeminate beauty, charm and dash become icing on the cake. Field slips so effortlessly into the role of the elderly Miss Trotwood that she's almost unrecognizable. But don't worry that her American accent will betray her in a cast full of talented Brits. In this production, she sounds as though she might have been born in England's Lake District. And Richards struts his stuff to the max as Micawber, one of the two juiciest roles in "Copperfield." But the tastiest role of all for any actor must be that of the ingratiatingly evil Uriah Heep, played in the TNT version of Charles Dickens' classic by Frank McCusker. He struts, preens, whines, raves like a lunatic and simpers with vigor and élan. "David Copperfield" is a story that has enchanted readers for 150 years. It has long been a favorite of filmmakers, as well. Audiences delighted in the 1935 U.S. movie version and in the 1969 film version starring Edith Evans and Laurence Olivier. But two excellent TV versions in one year? That might be thought of as too much of a good thing. On the other hand, it's a chance for fans of the story to revel in and compare two versions of an old favorite. Sometimes, as in this case, too much is just enough.