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I can think of a few reasons to do it each way, but I'd like a complete list of them.

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up vote 3 down vote accepted

Advantages of full install via iTunes:

  • Device will be synced and backed up automatically during the update process
  • If at location without Wi-Fi access (and without an unlimited data plan), won't use up (large?) portion of the monthly quota
  • Device will be charging during the update

Disadvantages of full install:

  • Tethered to computer for the duration of the update (unless using Wi-Fi Sync)
  • Full update could be significantly slower due to the large upgrade file download

Advantages of delta update via Settings:

  • Convenient - no need to be tethered to a computer while updating
  • Delta updates are smaller - with potentially shorter total update time

Disadvantages of delta update:

  • Users might not back up devices if prior to running the delta update - a potential issue if the update fails
  • Update process might use significant portion of the device's charge on the battery, running risk of corruption if the battery dies during the update
  • If device isn't at 50% power, the device can't be updated via the update process unless plugged in (per this article):

To install these updates, however, you need to have at least 50 percent battery life on your device, or have it plugged into a power source.

I'm sure this list isn't comprehensive, but it contains most of the issues that I can think of.

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The backup is, to me, the important piece of doing it via iTunes. And, I have a 64 gig iPad with a lot of ripped movies on it so backing everything up, installing the new iOS version, then restoring takes some time. A cable has been a reliable way to do it. – Richard Mar 9 '12 at 3:32
@Richard, agreed - I always do a backup prior to an update, whether or not I run the full install or the delta. Would hate to reinstall from a very old (or no) backup. – JW8 Mar 9 '12 at 4:09

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...

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