Here’s a nice article on choosing between mail apps and add-ins for Outlook development:

http://blogs.msdn.com/b/officedevdocs/archive/2012/12/10/when-should-i-choose-to-create-a-mail-app-versus-an-add-in-for-outlook.aspx

Bottom line: if you can do what you need to do from a mail app, it’s the way to go, especially since apps developed for Outlook will work in OWA..