Over at Grist, Dave Roberts has a great post asking whether the Senate climate bill is really, as today's Post describes it, "more ambitious" than the Waxman-Markey legislation that passed the House back in June. It all depends. The Kerry-Boxer bill in the Senate does have slightly stricter near-term emission targets and scrutinizes carbon offsets more carefully (at least, the initial draft does—we'll see what happens after the full Senate gets its paws on the bill). Those provisions, on their own, will likely mean steeper cuts by 2020 and fewer bogus offset projects.
On the other hand, Dave's right that the House bill has stronger provisions for energy-efficiency upgrades and bolder mandates for renewable power. (According to an EPA analysis, the Senate's efficiency program, which was passed by Jeff Bingaman's Energy Committee in June, does about half what the House climate bill would achieve.) The Senate bill takes a different route in addressing methane, which is subtly important—see the wonky footnote below.
Now, I disagree with Dave that the weaker efficiency measures in the Senate bill will affect the overall amount of carbon reductions. Under a cap-and-trade system, efficiency programs won't lead to additional cuts over and beyond what the cap requires1, they'll just enable companies and households to meet the targets at a lower cost. (Using power more efficiently and less wastefully is often the cheapest way to tackle carbon pollution, but there are frequently market barriers and misaligned incentives standing in the way of doing so, which means that government mandates can have a surprisingly large impact.) Still, lower costs are important, so this is worth keeping an eye on going forward.
1A wonky footnote. Dave cites this analysis of Waxman-Markey from the World Resources Institute. This chart, in particular, gets posted a lot, and I think it gets slightly misunderstood:
Notice that, in the near-term, WRI estimates that the cap-and-trade portion of Waxman-Markey would reduce U.S. greenhouse-gas emissions by a fair bit, but most of the heavy lifting would come from "complementary requirements" (that dark blue line). This isn't referring to the energy-efficiency and renewable-energy components of a climate bill. Since those two things mainly apply to capped sectors like electric utilities, they wouldn't lead to any additional cuts.
Instead, that dark blue line refers to complementary regulations on sectors not covered by the cap-and-trade system, like methane from landfills, coal mines, or leaking out of natural-gas pipelines. (Everyone saw that New York Times piece on the scourge of invisible methane leaks, right?) Oddly, the Senate bill doesn't explicitly regulate many of these uncapped sectors—it would instead make them eligible for carbon-offset projects—which is one major difference between Kerry-Boxer and the House bill, though I'm not sure what exact impact this would have.