I think that if we are seeing the history between barcelona against espanyol and we will see that if espanyol has been even drawing against barcelona in so many match. Espanyol may have a small chance to win against barcelona but it has a very big chance to force barcelona to lose two points from the match caused by draw.
The different result with head to hear between real madrid against valladolid. It seems like that real madrid will be facing easy opponent compared with barcelona. It's known that if espanyol was forcing the match against barcelona to be drawn so many times.
The Barcelona-Spanish derby has not only a sporting rivalry, but also a political rivalry. Barcelona has historically supported pro-independence stances, who want a Catalonia of their own, through a referendum on independence. On the contrary, Espanyol, as its name suggests, supports the view of a Catalonia as a part of Spain that should not be given the right to independence. This adds extra tension to the party.
As for the possible outcome, I think, as you say, that Barcelona are most likely to win, but it would not be surprising if Espanyol could snatch a point, so that Barcelona's lead over Madrid could be reduced.