The issue of peak excellence vs. longevity is a difficult one that Bill James used to write a lot about for baseball player comparisons. Mantle was unquestionably better than Mays at both players' peaks, but Mays was great for much, much longer, and on balance had the better career. Take your pick.
I think, though, that I agree with Rich Shefchik. Tiger is doing what he is doing at an amazing rate against vastly superior depth of fields overall. If he quits now, I think that his peak is significantly better than anybody else ever, hands down. Not even close. That doesn't diminish Jack, just like Jack doesn't diminish Arnie in my mind.
By the way, for those who say that nobody steps up against Tiger, remember the crowning moment of Jack's career, the '86 Masters. It was an incredible, unbelieveable win by an icon of sports, but don't forget that Ballesteros, Norman, and Kite all spit the bit coming down the stretch, or it never would have happened. Great ones have that impact on lesser players, and Tiger is no different.