From an optimization point of view, what optimizes different resources better? Activating/Deactivating UI elements or hiding and showing UI elements using CanvasGroup's alpha?

Do you want to optimize CPU cycles or memory?

Neither deactivating UI elements nor setting CanvasGroup.alpha to 0 will free up much memory. To do that, you'll want to actually unload the UI elements from the scene.

Also, be aware that (re)activating a UI element will cause it to recalculate layout, as well as meshes for Text components in all children, which can cause a stutter on that frame depending on the complexity of the UI. When optimizing for performance, this is worth considering.

Another alternative to setting a CanvasGroup's alpha to 0 is to disable the CanvasRenderer. Both of these techniques keep the layout in memory so it won't have to recalculate. But they also both keep the UI elements accessible to the GraphicRaycaster for selection by the pointer, and any scripts running on the UI elements will continue to run, unlike deactivating the GameObjects.

The Optimizing UI Controls and Other UI Optimization Techniques And Tips pages contain good information about this subject.