Here is the tutorial on how to erase iPhone phones: http://www.safewiper.com/tutorials/erase-photos-from-iphone-before-selling.html
You can also...
Your concern is necessary because there are many data recovery programs available to retrieve private data from sold devices. I suggest you to...
iCloud is the best friend to backup iPhone contacts to computer.
Back up the iPhone videos to a computer and import them into iPad later.
You can sync with iCloud and export contacts from iCloud.
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