First it should be noted, that at the time of writing this, the XML sitemap module is not working on Drupal 8, which makes Simple XML sitemap the obvious choice for Drupal 8 installations. However, there is effort to port the XML sitemap module to D8 stable, see this queue. This comparison may be interesting for XML sitemap users moving to Drupal 8 or for users intending to wait for the port. (Do not wait, help out!) Please also be advised that gbyte made Simple XML sitemap, which makes this comparison intrinsically biased.
There are major differences between Simple XML sitemap (simple_sitemap) and XML sitemap (xmlsitemap) and depending on your use case, you might want to choose one or the other.
What sets the modules apart are their complexity, performance and feature sets.