The best answer here is your last choice: evaluating one editorial's argument and then the other's. The reason for this is fairly simple. If you're writing a comparison essay of the two, you can't effectively discuss the editorials without an understanding of what they're discussing. So, you first need to develop that understanding by reading and evaluating whatever it is about.
Once you've done that, you can start the essay, but you have to make sure you evenly discuss both. Because your writing should be non biased, you have to equally present both sides or both editorials to your readers without influencing them by choosing one over the other. If you didn't present each side equally, it would then make your writing non credible and you most likely wouldn't be taken very seriously.
