i think it will be similar to that buffy episode where she's stung by a demon and ends up in a mental home - she doesn't know which of her realities is real
i assume the dream lord will do something similar - one reality where they are all happy, amy pregnant and married etc, and one where the tardis is about to be destroyed
they have to decide which reality is the real one
amy's choice is that she chooses not to live in the dreamlike state (happy, pregnant etc) and chooses to live in reality..?