the iphone hands down for me. They are better integrated and just work. Most of the android phones i've tried (never actually used an s4 though) had odd quirks about them. Also for me the s4 is just too big for a phone. If i need a bigger screen I use a tablet.
I think it's all about what you want in a phone and what you might be use to; because my first phone was the iPhone. I tried switching to the S3 and I didn't like it at all and now I have the iPhone 5 and I love it. Plus, it all depends as to why you want to use your phone as to what one might be the best for you. Everyone knows that customization is better on the Samsung but the iPhone 5 also has many pros to it plus I like the size and feel of the iPhone 5 better then the S4. I find that the performance is better on the iPhone 5.