Internet Mail provider does scan mails from full Outlook in the situation where the Outlook / Exchange provider is either uninstalled or (perhpas) just turned off.
You may choose whichever provider you like best. Outlook / Exchange is more suitable for full Outlook, it does not have problems with timeouts, can scan emails coming from SSL secured connections, can protect you from infected emailes already in your .PST file and the like. It costs you nothing when Outlook is not running (no memory or CPU time).