Home runs per at bat. It isn't even close. Hammer has more power than anyone on the team besides Dunn. He just isn't a good hitter.
If you define power as HR/AB then sure, but Dukes is probably one of the strongest players in baseball.
Besides, without looking it up, I'd be willing to bet that Willie Harris has more HR/AB that Duke over a few 6-week stretches, but that doesn't mean he's got more power.