Spuzz
They're the two best golfers of the modern era... both well into the sunset of their careers, granted. And although both have indicated that their rivalry isn't nearly as intense as it was a decade ago, as they've reached out to each other over the past few years and become more friendly - I think Phil and Tiger still see each other as rivals and would love nothing more than to beat the other's brains out on the golf course.
I think there's still a lot of pride and ego there, enough to make this interesting.
Jack and Arnie were always trying to 1-up one another, well into their senior years. Tiger and Phil are the Jack and Arnie of our generation basically. We've not got to see them go 1-on-1 very often during their careers, and I'd be interested to see how their games stack up under that type of 1-on-1 competitive atmosphere.