If your goal is long-term reduction in CO2 emissions, you've introduced market volatility. The goal of any such measure is to reduce CO2 emissions, but the -mechanism- by which it does so is encouraging research into alternatives. If the market is volatile, the value of any such research is called into question; tomorrow it might be valuable, it might be worthless. A tax, by comparison, has a fairly static value. The cap-and-trade measure, through its volatility, increases the risk of investment into reducing carbon emissions; the value of your investment isn't determined by the degree to which you can reduce carbon, but by the amount of carbon emitted in the market as a whole.

Unless we assume some level of carbon emission is better than no carbon emission, the tax scheme is better.