I can think of a few reasons to do it each way, but I'd like a complete list of them.
Advantages of full install via iTunes:
Disadvantages of full install:
Advantages of delta update via Settings:
Disadvantages of delta update:
I'm sure this list isn't comprehensive, but it contains most of the issues that I can think of.
I agree guys I keep making the mistake with 64Gb Iphone 5 and Ipad Mini. I see the update in the setting and run off and do the update over wifi relying on my back up in the cloud. I forget if it fails the full backup over the air is very laborious, if it even works (especially beta) or it will be very very slow. I too would recommend full back up to your local computer via itunes and then diasble wifi/cellular and do the back up over cable through iTunes. I just need to follow my own advice...