I doubt that Sondheim would have personally called Catherine Zeta-Jones if he didn't think that she could do the role. Sure, he probably was "convinced" by Trevor Nunn and the producers, but I'm sure that Sondheim wouldn't let himself be bullied into a casting decision. He has final approval over all casting of his broadway revivals. Clearly He thought that Zeta- Jones could do the role and sing "Send in the Clowns" effectively, and she has proven him correct. My advice is to see both, if you can.