I say Prince. The MIL GM has talked about it is easier to get full value for a hitter during the off-season vs. in a mid-season trade. Not to mention, there was talk with Overbay moving to the OF. While Prince has not torn up AAA, he's done pretty well for a 21 y/o, and I think with Weeks and Hardy already up, MIL will continue to make their transition to the youngsters next year. Heck, they let Hardy play even with all of his struggles, and it isn't like Fielder stunk in his call-up.
As for Jackson, ARI still has to figure out what to do with Tracy, Clark and Quentin, and trading one of Green, Gonzo or Glaus is not an option - and none of these guys would be a natural CF. So, while I think Jackson is in ARI next year, unless they get rid of several guys, his PT would appear to be more sporadic.
Plus, on pure upside alone, IMHO, Prince has the highest upside. Not sure how long it will take him to get there, and not sure how long you can keep him, but for those reasons, I'd take Prince.