Better prospect and more upset are 2 different questions.
Better prospect in strictly baseball terms, Reddick and it's not even close. Arguably the best OF arm in the minors with good speed. Should be an everyday OF for many years. Great hitting approach. Can be a high avg guy who will walk, hit for a little power and may develop more.
Johnson is strictly a HR/K guy. Because of his raw power, he has a higher upside, but he's very far from realizing that potential. There's a very strong chance he flame outs considering his ridiculous K rates at low levels.
So the question is how deep is your farm team and how much to you like to gambooolll? To greatly overysimplify (I also hate comparisons), which would you rather have:
Reddick: 75% chance he's a Shane Victorino type with a better arm, 25% chance he's a slightly better Corey Patterson type
Johnson: 5% chance he's Ryan Howard, 15% chance Jeff Francouer but terrible defensively and probably a 1B, 80% chance he never makes it above AA.