This question might have been asked and answered but I would say CO2, absolutely. This article details a scientific test that pretty much states what m00kfu said, in that the rats appear to suffer [B]more[B] when using argon gas than with CO2. The article itself is fairly dry and is written in scientific terms but it has some good information if you wanted to check it out.

Here's the link: http://www.ncbi.nlm.nih.gov/pubmed/20819391

Good luck, whatever you decide. Hope that helps.