There are a few trends to support the Astros here, but the pitching looks like a real mismatch, so I will go with the Cubs behind Wood.
The Cub's Kerry Wood is excellent against Astros, 6-1, 2.08 ERA, 0.881 WHIP! Astros are 6-17 vs. a starting pitcher whose WHIP is 1.250 or better this season (Wood is 1.203 on season). Wood is also 10-3 the last 3 years when coming off 5 or more days rest.
Astro's Wade Miller is struggling this year with a 7.92 ERA. Cubs are a team Miller struggles against anyway, 1-2 with awful 7.31 ERA and dismal 1.875 WHIP.

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Good luck!