I'm not sure you can compare their minor league numbers for many of the reasons mentioned here.Context matters and they don't have much in common. That said, Dozier has shown the ability to be an elite hitter and above average defender at the major league level for long periods of time. While Polanco to be fair hasn't had the opportunity to do so, those types of players don't exactly grown on trees.
I think at the end of the day, trading Dozier is the right answer, especially with only 2 years left under contract, but I don't pretend to think that Polanco can walk in and out produce him in 2017. He may never put a season like Dozier.
You are correct that Polanco hasn't had his opportunity and we all know that has to change. And soon!
I'm still torn on trading Dozier. He still brings a lot to this team, but his return should also be good. And he's still young enough to be part of this rebuild.
I find the Polanco vs Dozier debates to be interesting. I see no reason why Polanco can't be a fine defensive 2B, as good as or better than Dozier. But they are different types of players. And that's OK. Polanco may develop some double digit HR power, but will probably never be a 20+ HR guy like Dozier. But he will almost assuredly hit more, have a decent to good OB, and provide speed and quite a few doubles. Losing Dozier's HR would stink, but it should also be offset, eventually if not immediately, by 1B/DH becoming more settled, Rosario and Kepler.