ok, if you are going to be writing notes, the metro is more user friendly but it does not have that many options available compared to the OneNote Desktop.
OneNote Desktop for free does not include all the options and features that comes in OneNote paid version. Some of the features missing are the possibility of attach voice notes and attachment of files to your notes.
OneNote Metro automatically sync your notes with OneDrive and OneDrive with your OneNote Desktop. So whatever you create in one of them is available in the other one if you are connected to Internet.