What's More Likely: That Your Vote Will Matter or That You'll Help Discover Extraterrestrial Life?

Here’s an e-mail from a reader named Nadaav Zohar of Akron, Ohio. I like the way he thinks.

Every election season, I can usually count on a Freakonomics blog entry or three about voting and why it is pointless. I very much agree with your analysis, and I don’t vote.
However, I was wondering if you would also not participate in SETI at Home.
The chances that the power boost provided by your idle computer would help the SETI project discover something useful (the jackpot being extra-terrestrial life) is probably about the same or greater than the chances that your vote would decide an election. And the cost (10 MB on your hard drive and any unused RAM) is probably about the same or less (no waiting in lines, no researching issues, etc.).
The potential payoff of SETI is definitely better than the potential payoff of voting. Even if the election goes your way, you get another politician, another stupid law, etc. But if SETI finds something useful, we have revolutionized everything we know about life in the universe.
What do you think? Is SETI at Home worth it?