Well, let me tell you why I disagree and say Mills, even with the liklihood that he moves to 1b because he profiles as a Ryan Braun-ish fielding thirdbaseman(just kidding, not really that bad, but surgery to his arm hasn't helped matters). Parker, as a 2007 draftee, is at the very least 3 years away from helping the parent club, and besides being smallish (6-2 and 175 lbs), needs to fill out in order to be able to be anything more than a reliever. Not doubting that he will, but the liklihood that the whippish action his arm has used to generate so much velocity(scouts love it) has come at a cost. Additionally, hitting prospects are typically easier to project, because of the huge injury risk pitchers face that has been talked about ad nauseum over at baseball prospectus.
Either way, both are great prospects, and I hope the guy you select wins you a lot of games.