Attention seeks to measure how long the potential customer was paying attention to the ad. Or engaging with the experience.
Session Duration aims to capture how long overall a customer was engaged on the website. However it has limitations, such that, if you swap tabs, then resume, it measures all that time in-between. It also typically ends a session after 30 minutes of no activity. Combined, these mean metrics like session duration tend to overstate reality. Wheres Attention is far more accurate.