The differences between these 2 plugins:
- Polylang is a free plugin. WPML is a premium (paid) plugin (and you need Multilingual CMS version).
- If using Polylang, you need to translate theme strings (included in the .pot file) by using Poedit or Loco Translate. If using WPML, all the text can be translated by using WPML.
- When using Polylang, if you need to make your WooCommerce shop to be multilingual, you need to purchase their Polylang for WooCommerce Extension. When using WPML, the feature is already included when you purchasing WPML.
- And there may be some other differences.