You still have to check if everybody has paid their emission tax.

Also a tax has to be high enough to offset the average cost of lowering Co2 emissions (since you force everybody to lower their emissions), whereas in a cap-and-trade system the price will the the lowest cost to lower the emissions, since anybody who would pay a higher price will be better of buying the emission permissions and anybody who can save co2 cheaper will be better of selling them.

Thus a cap-and-trade system is always going to be more efficient.