There's no one metric to A/B test anything really. Conversion rate may be the best metric for ads/sales pitches, but even that's often an oversimplification.
Instead, look at all your metrics and see what needs work. Do you have a concerning bounce rate? Is time on page low? Are users not clicking the links you want them to click? Trying to improve an already low bounce rate is a waste of your time, and might be extremely frustrating as you find there really isn't anything wrong.
Normally you should create designs based on whatever you feel needs improvement, but since you already have both designs, I'd just A/B test both of them and keep an eye on as many factors as you can; bounce rate, time on front page/articles, which links get clicked, anything you can. Judge the two designs holistically. A Site with low bounce rate on the first page but high bounces on the second level (because users were misled/confused) could actually be an indication of a worse design than a landing page with high bounce rate (because people, correctly, realize they're legitimately not interested in the content, which is clearly defined).