Auction league, or draft? If auction, what values do each player have, if draft, what round?
I think you'd probably get better production for this season out of Zobrist over Polanco, so I'd probably try to weigh whether you can reacquire one of them after you let them go and figure out who gives you the most value. Zobrist is a top-30 hitter, but his average draft position on Yahoo! is the 16th round, so you could probably just let him go and redraft him. Polanco's ADP is the 10th round, so assuming you're keeping him at a lower round than that, you're getting some good value for him.