When comparing the two players, i have them ranked pretty similarly, but when it comes to deep leagues, position players get the higher value over pitchers, especially at such a scarce position like SS. I wouldn't take the Cain for Drew trade in that regard.
However, another pretty scarce position (not as much as SS though) would be 3B, and I could make a case for trading Drew for either Sandoval or Alvarez. I personally like Alvarez, i think he's too talented to continue on this slump and is primed for a turnaround. He has lots of upside, and with a young castro at SS as well, you could perhaps afford the immediate, although slight, dropoff at SS.