A Cap and Trade system would provide economic incentives towards companies in reducing carbon emissions as opposed to penalizing them. A carbon tax would have to be significantly high enough to prevent the expulsion of high amoungs of carbons, and most companies would rather take the tax and penalty and still create a profit. With the Cap and Trade system, the companies would get incentive to not produce as many emissions and thus give incentive to not waste.
A cap-and-trade system communicates the message that, for corporations that can afford it, any level of emissions is acceptable. Cap-and-trade allows big polluters to offset the damage they do by trading carbon credits. A Carbon Tax would deeply impact the bottom line of the corporations with the worst pollution track records. It would hold every business equally accountable, and help promote the perception that the damage we do to the environment comes with an economic cost.