Hi Hellwitu,
For the majority of purposes, you’d want to use the Merge Actors tool. It’s got many more robust features that allow you to remove unseen vertices/faces, merging the physics assets of the actors, merging textures into atlases for better optimization, etc. The Convert Actors to Static Mesh is good for quickly grabbing an actor from your level whether it’s a static mesh or skeletal mesh and converting it into a static mesh. This can be a really cool feature for capturing some skeletal meshes in a play state with mid-animation and converting this to a static mesh to capture the action. Before this particular feature to get a static mesh like this you had to use the File > Export Selected and save as an FBX to get this. Now you can simply do it in the editor.
Overall, the Merge Actors tool is the way to go. If you need something quick or need to convert a skeletal mesh to a static mesh the alternative is your option.
-Tim