Cap 'n' Trade or Tax?

What do you think? According to economists, both can be efficient. The Democrats have unveiled their proposal, a cap & trade program, presumably assuming that it would be more palatable to the business interests whose support they crave. In today's New York Times, Tom Friedman argues that a tax is better, since it's more forthright and it will lead to innovation. I like the idea of having it presented as a matter of national security, and Friedman's right that it's still basically going to be a tax. Still, I'm not opposed to the cap & trade program. What do you think?