Matt:
Good thread; I, too, think Casper holds the edge, as of now, although I'm fairly certain Lefty will win at least one or two more majors along the way.
I tend to look not just at major wins, but also whether a player has been competitive (top 10 finishes, runners-up) in a major, in assessing players. Casper never quite got over the hump in the PGA, runner-up three times (although never really close to Jack in '71), with five other top 10 finishes. He was really consistent in the Masters, and played a number of US Opens fairly well in the 60s and 70s. He was one of those pros who disdained traveling overseas for the British Open (not an uncommon occurence in the 1960s, despite Jack and Arnie's efforts to promote it by playing there), and played it only a few times during his prime competitive years.
One of the real premier putters in the game.