oeman50 is right on, but I would strongly suggest that you investigate whether your mixture is in the explosive range before having anything to do with this project !!! You need to consider possible process upsets, including startup and shutdown conditions as well. Even if this is in the explosive range, there are ways of doing it safely, but they are complex, and if only to cover your butt [legal liability] you should hire an expert.
I would say the most economical is carbon steel, as long as it is DRY. Having water vapor in the mixture can allow all sorts of corrosive materials to form. The chlorine is your determining factor here, since ethylene is not nearly as corrosive as Cl2. Carbon steel is good up to about 400F (~204C) with Cl2. It may not have the best heat transfer properties, but its cost will easily allow you to design in more heat transfer area and still come out ahead in price.
