What is a better measure of evaluating social media performance? Engagement total or Engagement rate (%)?

You should do your best to always apply a metric for SMM by looking at the click through rate. This is, by far, the best way to help you evaluate whether or not your Social Media Marketing campaign(s) is/are going the way you expected. When you see how many people are or are not clicking through to your website, you can determine how to alter your CTA (call-to-action) in order to make people more intrigued to follow through to your site (if the current method is failing, of course).