I use both but prefer Outlook if for no other reason than the way it handles/places attachments at the top of the message under the subject.
This is also such a nice feature of the Outlook app on iOS which other major e-mail apps lack (e.g. Apple Mail, Spark, Airmail, Boxer, CloudMagic). There is BlueMail/TypeApp, Timyo and MyMail which also offer this but they lack a lot of other features. See also my e-mail comparison for iOS/Mac here.