These are very different lasers and are difficult to compare. The ideal study would be one where one half of the side of the face is treated with one laser and the other side with the other laser. This is unlikely to happen in a IRB approved study due to the inherent differences in the lasers. What you should know is that they are different wavelengths and so their properties are not the same. It is well known scientifically that erbium yag has a higher affinity for water than does CO2 and so it can specifically target the cells you are treating with less collateral damage. That is to say that although both lasers could be set at levels that can scar and burn people, or even levels that you would barely feel, the erbium yag is more efficient at removing tissue precisely. What is best is to keep in mind is that, in the right hands, nice results can be achieved with either laser. The downtime, cost, pain, number of treatments, risks of scarring and pigment changes are different with these lasers and should be discussed with your doctor before treating. It might even be best for you to meet with one doctor with one laser and another with the other laser and ask them the same set of questions, compare prices, amount of pain with the treatment, risks of scarring and discoloration of the skin, downtime for healing and end results. Always ask to see pictures if possible. Preferably ones that are not from the laser companies but rather their own patients. Then ask as many questions as you need to be comfortable you are with 1. the right doctor 2. with the right laser 3.and have the right expectations for your treatment.