The correct answer is "neither"

Carbon tax vs. cap-and-trade | Bulletin of the Atomic Scientists

To cap or to tax? That is the question, at least for U.S. policy makers intent on reducing the nation's carbon footprint by making emissions costlier. There are impassioned advocates for both systems, but upon closer inspection, cap-and-trade schemes and carbon taxes can start to look like two sides of the same coin. Our experts discuss the benefits and drawbacks of each, and pinpoint the real choices policy makers need to make if they want the market to get to work on global warming.