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Some OneNote functionality can be duplicated in Docs, but OneNote itself is quite powerful with cloud integration built in.
...Office's are a wee bit more sophisticated.
Unless you pull something funky with tables or CSS, you're locked into one kind of spacing in a Docs document. Obviously, you have your choice by paragraph of spacing and fine formatting in Office.
See also: Christopher Dawson's Office 2010 vs. Google Docs updates