There are several different types of XML editors. Each has advantages and disadvantages. The primary criteria for selecting an editor is the purpose for which you will use it.
WYSIWYG Editors: What you see is what you get. You edit content exactly as you want it to appear. Their focus is more on the final appearance of the document.
WYSIWYM Editors: What you see is what you mean. You edit content focusing on the meaning of the data as expressed in the XML code. Their focus is more on the meaning of the data rather than the final appearance of the document.
Text Editors: What you see is ... well, text. You edit XML using a text editor like Notepad.