I respectfully disagree. Other golfing greats (Palmer, Casper, Player, Miller, Trevino, Ballesterios and Watson) put constant pressure on Nicklaus and he more often then not came out on top. Tiger has fallen in head to head battles with the likes of Rich Beem, Zach Johnson and Anjel Cabrera.
Look at a tape of the 86' Masters - Nicklaus made everything - and then look at a tape of Tiger in this year's Masters or U.S. Open where he didn't make anything.
Tiger is a great frontrunner and bully, but he rarely wins the close ones like Jack did. I think Tiger will surpass Jack eventually as the greatest player of all-time, but certainly he has a long road to go to prove he's better than Jack when another player applies the pressure.