The plan is that the GSoC work will make if in, but its under review right now and has been for several months. It will likely make it in, its just a matter of when.
Both systems offer the same functionality but through different ways. I fully plan to support the plugin in Godot 4.0 once it’s released, regardless of whether the GSoC work is integrated or not. I plan to continue to work on the plugin and add new features over time.
That said, the IK system in Godot 4.0 is official and will be supported by the Godot community, unlike this plugin is only supported by myself, so over time the Godot 4.0 system may be more worked and mature due to the potential of more developers working on it.
Anyway, all of that is I cannot say for sure nor can I suggest one system over another. If you want to only learn a single system and am wanting to use the official solution in the future, then I might suggest waiting for Godot 4 as the plugin is different in implementation.