The way I see it, there's no immediate need to trade either Santander or Mancini. The O's have a lot of good prospects at AA and below, but they don't have any OF/1B types at Norfolk who are making a compelling case that they need to be called up. Maybe Yusniel Diaz or Johnny Rizer will force the issue later this season or next season.
You could make a timing argument that Mancini will never be more valuable than he is now, but I believe that he's a very good hitter who is likely to look like a very good hitter this time next year, or over the offseason. Trading Santander right now would certainly not be selling high.