If you plan for Buxton to be hurt, you have to go get a CF......if that's how you think Polanco plays.....
He should be traded for prospect(s), preferably SP or C. I don't know how you PLAN to not play Buxton during the spring.....and it doesn't make sense to have one of Polanco or Julien on the bench, not when you can turn one of them into assets that can be on the field at the same time as the other one.
Given the team's constraints now and in the future, it makes sense to keep the less expensive one, the younger one.
From an asset management and 26 man perspective, it just makes sense for one of them to go.