What are the difference between net.wasdev.wlp.maven.plugins and io.openliberty.tools liberty-maven-plugin and which one should I use ?
The io.openliberty.tools one is the newer, better version of the liberty-maven-plugin, and the one you'll likely want to use for new development.
The older net.wasdev.wlp.maven.plugins plugin is what evolved into the newer io.openliberty.tools plugin.
Along with the move from version 2.x to version 3.x of the liberty-maven-plugin, the groupId was also changed from net.wasdev.wlp.maven.plugins (versions 1.x-2.x) to io.openliberty.tools (version 3.x and beyond).
Both versions of the plugins can be used to work with (install, deploy to, etc.) either Open Liberty or WebSphere Liberty servers. (so that is not a concern in choosing one vs. the other).
The newer V3 plugin provides the helpful new "dev mode" support. Plus you'll find a lot more recent material using this version such as the Open Liberty guides.
It also enhances and streamlines some of the other goals and configuration (besides "dev mode"). There's a summary of the 3.x enhancements here.
So at this point the only reason to use the 2.x plugin from net.wasdev.wlp.maven.plugins would be to continue with a project already using this version of the plugin (unless or until you're ready to migrate).