Its a really personal prefrence, Android is more customizable, more hardware options, can be cheaper, and generally you can do more advanced things with it.
However IOS has a much stronger and higher quality software development community (apps), generally easier to use (if someone wants a simpler smartphone), Generally from my experience is more stable from the lesser amount of hardware fragmentation.
I've owned both, both have their ups and downs, currently running a nexus s though had an iphone 4 before that and really did like it alot as well.