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I prefer an external skimmer and external pump. By using true union valves, it's easy to remove the skimmer or pump for cleaning and maintenance. Plus, I tend to get less water over everything that way.
That being said, any of the methods work just fine. The difference mostly depends upon the size tank the skimmer and return pump are being used on.
An in sump skimmer and return pump are just fine for smaller tanks. However the pumps used in a system like this are more or less glorified powerheads. Something like that is not going to be able to process enough water on a larger system.
If you've got the room (and budget) go with both external pump & skimmer. The main advantage as I see it, is ease of maintenance,
which faciltates more frequent cleaning.
That said, ALL of the combos you list will do the job provided the skimmer and pump are properly sized for the type and size of the aquarium.