I have an upgrade soon, and here is the reason I'm going from Android to iPhone. (Eligible for an upgrade right now, but waiting for iPhone 5s.)
All my friends have iPhones. While you s4 supporters are probably thinking "Be different, don't follow the crowd" I've learned from my mistake before. 95% of my highschool has iPhones which means I am contantly missing out on group chats, and my lady friends get mad at me because they send emojis and I can't read them.
I know the S4 will have specs that are far better then the iPhone 5s whenever it gets announced. That is why I got my current, Evo 3D phone, because it was the first dual core phone ever, and its processor was way better than the iPhone 4 that was out at the time.
Apple always gets the apps first. My friends are always playing games vs each other on their iPhones, and I can't join in because I'm on an Android which doesn't have those apps.
I also like how i can just plug in my iphone to my computer, and it will instantly sync all my new songs and other stuff. It is kind of annoying having to plug in my evo, set it as a disc drive, find the music folder then drag in my music. Kind of un-organized and annoying.
The s4 beats the iphone 5, and will beat the 5s in every aspect. Better screen, better processor, more ram etc. But the iPhone is the most popular, so you will always get apps first, and have access to the most accesories. Just my opinion.