Spoiler: They're both excellent teams. But the Giants will have to play one of them. It'll probably be the Reds, as the Nationals need to win just one more game to secure home-field advantage (they hold the tiebreaker), and the Reds would have to win out against the Cardinals.
The real answer is "Astros." I think some lawyers should look at this thing and see if there are any loopholes or injunctions. These will all be billable hours, folks, so see if you can work up some writs. All the writs.
But this is the kind of tomfoolery that you get before the regular season ends and before the playoffs start. It's pure, meaningless speculation, and it has a short shelf life. So let's dig in!
Giants' record against in 2012
Well, that's no fun. The Giants played a combined four series against these two teams, and they didn't win a single one. But the Nationals whomped on the Giants much harder. The Reds outscored the Giants 21-28, whereas the Nationals outscored the Giants 45-24. It was the Nationals who were in town when Melky went back to his home planet, and the Giants played their last game against them in a fugue-like state.
Deciding factoid: The Giants were outscored by the Braves 21-28 in seven games during the 2010 regular season -- the exact same differential they had against the Reds this year. If you squint, you can compare Bryce Harper to Jason Heyward, and Chad Tracy to Eric Hinske. Yeah. There are no coincidences in this game; there are just random, unrelated things that appear to be linked in your withered, apelike brain.
Johnny Cueto is good because he doesn't allow any home runs. That's cool. The Giants won their division even though they don't hit them -- they have the fewest home runs of any playoff team since the 1987 Cardinals. I included that factoid to make you want to punch something. Jose Oquendo is still around.
Gio Gonzalez is good because he strikes out a lot of the hitters he faces. When he gets in trouble, it's because he can get a touch wild. That's not cool. The Giants will finish the season below the league average in walks, so maybe they're not going to match up well with Gonzalez.
On the other hand, the Giants have a chance to finish with the lowest strikeout rate in the National League. Balls in play ahoy! So maybe Gonzalez is the better fit after all.
The rest of the rotations pan out sorta similar. I don't want the Giants to face either Jordan Zimmerman or Mat Latos, and while I'm a little bit more dubious of Edwin Jackson and Bronson Arroyo, both are capable of good games. Ross Detwiler is a lefty, and the Giants hit lefties pretty well, but Homer Bailey would probably start his game in Cincinnati, where he's usually terrible.
But I'll go with the Nationals because they don't have Stephen Strasburg. It's a psychological thing. More for me than the Giants.
The Zito Factor
If the Giants are really, really, really committed to starting Zito no matter what, this one's easy. The Reds do much better against left-handers. The Nationals are pretty neutral. If Zito is going to start a playoff game, regardless of his opponent, here's hoping it's against the Nationals.
(Like it matters.)
XAVIER NADY IS THE X-FACTOR. I just came up with the nickname. But Nady played for the Nationals, and he can tell the Giants all their secrets.
Don't hang breaking balls to Mike Morse
You know, stuff like that. The counterpoint is that the Nats have Mark DeRosa, which nullifies that tactical advantage. DeRosa probably took binders full of information when he left. Well, he at least kicked them to his car when he couldn't carry them.
The real x-factor is Dusty Baker starting Miguel Cairo at first to give Joey Votto against a tough lefty in Game 4. Looking forward to something like that. /bunt
Conclusion: I would like the Giants to play the team that they can beat. So, like, whichever one that is, man.
But if I have to pick one, absolutely have to, I'll go with the Reds. The Nationals kind of humiliated the Giants this year. With the Reds this year, the Giants still had some good games -- the Angel Pagan ninth-inning home run game and the Madison Bumgarner shutout, for example. Against the Nationals, there was mostly misery. Add in the possibility of a comebacker off Mat Latos's shin, and I'm thinking that's the team.
Still holding out hope for the Astros, though. I'll settle for the Rockies. Help me out, lawyers.