Russell is right now, at 21, a better defensive SS than Castro by almost universal agreement. Whether he's going the be the better player is of course an unknown - if it were a lock that Russell would be what the scouts project, he certainly will be, but we don't know that. But the difference in defensive ability is reason enough to keep Russell at SS. It's not as though we're talking about moving a franchise player (which Castro is not) for a punchless defensive specialist. As to whether Castro is traded, that's hard to say. I think he's the most likely of our bevy of SS to be moved, but there are a lot of variables - what the potential return is, whether Bryant looks like he can stay at 3B, Russell's timeline, et al. The Cubs will be in a lot better position to answer those questions this summer, and if Castro is moved that will probably be when it happens.