Halladay has thrown 445.1IP over the last 2 yrs, with an ERA of 3.46 and a WHIP of 1.17.
Beckett's best yr was last yr, when he threw 200.2IP with an ERA of 3.27 and a WHIP of 1.14.
I'd take Halladay all day long. He has proven it over a longer stretch of time. If the Jays werent insane and let him throw 260IP a few yrs back, he may have had a few consecutive CY.
In terms of needing one game, with the season on the line, Beckett's my guy. But over the haul of a season, Halladay is the better bet.