The costs of CO2 - the real costs, not the subsidized ones - are extraordinarily high, possibly leading to the worst global catastrophe in human history. What is the cost of Cl2?
The pre-industrial levels of CO2 were around 280ppm, while the current CO2 levels are around 440ppm.
For Cl2, in occupational-health situations the permissible exposure is around 1ppm per hour, or 3ppm for 15 minutes. So the 160ppm we'd throw into the atmosphere in this thought experiment (or even the 52ppm at the suggested discount) is going to cause some pretty big problems pretty rapidly.
The problem with H2O wouldn't be the absolute amount, but the changes brought on by the added H2O. For one thing, it would rain pretty much 24/7 in Florida. :) (Yes, I know it's not at all that predictable.)
With the passing of time the answer might not be so clear.
The frogs in the 2 degree warmer pond will be complaining a lot more than the other frogs, so that must be worse.