At this point, I don't think there are any "must-win" situations(unless you're the Yankees, in which case seemingly every game is must-win nowadays).
Having said that, I like the Nats solely based on value. I got them at +136, and with Livan Hernandez on the mound, I put a unit on it just because I think that's good value. Perez has pitched well, and LA seems to be for real, but I don't like the value. Also, the Nats offense, while not scoring many runs in the series, have blown numerous opportunities with RISP this series. If they cash in tonight, they can win.
If anything I'd stay away from it. But if you have to put $ on it, I'd go with the Nats only because of the value. You could argue it either way. Perez has been great this year. But my $ is on the Nats. Those are my thoughts.