For cloud storage and back up I've used a mix of icloud, google drive, dropbox, Box & SkyDrive.
In general though I've tended to use Box more- as it's integrated into quite a few IOS apps on the iPad (as is dropbox). It also works well over IOS, android and windows. The clincher was that I got 50gb free for life with a Sony Z - and the ability to give 50gb free to a few other people (which wouldn't accept @hotmail accounts, but would accept @outlook addresses).
For photos it's a bit different- google drive and icloud I find easier.
Now that you can save gmail attachments directly into google drive my use of gmail for docs is picking up again.