I've used both and I like both.
iOS advantages (for me).
Tight integration with apple services and products. This cannot be understated for those who are deeply enmeshed with apple.
Simple yet elegant design.
Frequent updates, and no need to wait for the manufacturer to hopefully send an update.
Android advantages (for me again).
Customization the ability to customize the environment is nearly endless, whether we're talking about customized roms or just launchers.
Environment is less locked down and I can make changes if I wish.
A multitude of choices in phone selection.
I think either platform is great, and both offer strengths and weaknesses. I'm currently on iOS due to Point #1 but it won't take much of a push for me to re-embrace android. I'm liking the Note 3 phone and I may decide to take the plunge again