I'd go with S4. But it's as Wes said, just a matter of preference; I don't think there are lots of people that use their phones at their full potential, even though they buy their phones only if they meet a certain criteria(system specifications, size, color, etc.). Most people buy their phones to match the current trend.
iPhone has a more simplistic interface and OS, while Android takes a little while to master. When it comes down to specifications, Samsung wins hands down. However, I highly doubt anyone would notice the difference.