Which version of the plugin should I use?
As of January 2021, you should use the regular EWWW Image Optimizer as it contains our comprehensive image optimization functionality. There used to be two different versions of the EWWW Image Optimizer for WordPress. The regular one and a "cloud" version.
Previously, certain web hosts only allowed the Cloud edition, because there was some misconception about how EWWW IO operates. We have worked with these web hosts over the last couple years to clear up any confusion and to tighten up the security of the "core" EWWW Image Optimizer. The old Cloud edition will no longer receive updates and has been closed.
If you are still using the Cloud edition, you can get the latest updates by installing the regular version of EWWW IO, removing the Cloud version, and then activating the regular one. All your settings will transfer over and no data will be lost in the process.