Xili language is similar to qTranslate multi lingual plugin in that it does not create any new database tables, rather it mixes all the language content within the wp_posts table. By mixing all languages together in the database, you are reducing flexibility and extensibility.
WPML was originally ported over from Drupal, a system widely commended for its code design virtues. Drupal in fact does not operate in mixed content mode when it comes to multilanguage features, but separates things as WPML does.
Apart from this issue of separation, WPML provides a more complete solution for multilanguage websites. It can handle everything you would need to throw at it:
- Custom Post Types
- Custom Fields
- Theme strings
- Dashboard translation
Not to mention that it also integrates with the ICanLocalize translation services, through which you can have all your content translated using the translators managed by this company. The same people behind WPML also manage ICanLocalize, so you get a fully packaged solution to your multilingual needs.