This is a technical comparison of the older 2.x branch of the Simple XML sitemap module and an older development version of XML sitemap. XML sitemap has just had its first release and for more on the newer 4.x branch of Simple XML sitemap see this article.
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, extensibility, performance and feature sets.
Code base
Having been built specifically for D8, simple_sitemap has arguably a cleaner code base adhering to D8 standards i (i.e. use of OOP). In contrast, the xmlsitemap module will have a hard time adjusting to D8 technologies and guidelines, as it carries around a whole lot of legacy code going back as far as Drupal 5.
Performance
What is meant here is the impact of the module on a Drupal 8 system, how quick the sitemap generation process is and how long it takes for a visitor to fetch the sitemap.
Sitemap generation performance