Not enough info to make a decision.
They both have good reasons to pick them depending on where you are at in the draft, how the rest of the team looks, and what positions you have to fill.
If you have 1b, CI, Util, DH, then Hafner might be the way to go.
If you have a requirement for a large number of SPs relative to the total number of Pitchers, then Beckett might be the way to go.
You need to weigh everything else to make the decision.
NL West Winners - San Diego Padres