I would definitely go with Conch Key if you can get it over Boca Chica. We stayed at Boca Chica and paid LV prices. It was last Jan. and very low attendance and I specifically asked for castle view. Boy, was I disappointed. First of all, the patio wall was too high to sit and enjoy the view, you had to stand. Then we looked straight at the Contemporary. In order to see the castle, you had to lean over the railing and look way to the left and behind a palm tree.
I do think Boca Chica has some nice views, but your chances are much better at Conch Key.