If you want something you can hack around on without jailbreaking or paying for a developer's account, compiling the OS source (via Cyanogenmod, AOSP or others), change the default application for things like the browser, etc, then the S4 might be what you want. One thing I can say about Android is that I've seen it motivate and encourage quite a few people with no programming or hacker experience beforehand to go out and learn how to compile the source for their device, retheme it directly from the xml markup and pull up debug logs to give proper bug reports to the developers they follow. There's a lot childish behavior online and also in the Android modding communities, but it always makes me feel a bit better when I think of the people I've helped learn more about modding their device and how it works beyond what a typical user would care about. There may be a trolls and some sterotypical "man-childs" on XDA, rootwiki and other modding sites, but there's also many knowledgeable people that are willing to share what they know to anyone.
I know I got off on a tangent there sorta, but anyways, back on subject...
If you want something that works without a lot of modding and bloat, opt for a Nexus Device (either the Nexus 4 or the upcoming [though 600+ dollars] AOSP Galaxy S4). While it can be nice, the Samsung (Touchwiz skinned) S4 is going to have carrier bloat (a lot of useless junk) and could also have a locked bootloader that cannot be unlocked (if you like to mod stuff). Nexus devices are the closest Android equivalent to an iPhone as far as intended user experience by the creator of each OS.
I've only played around with my friends and relative's iOS devices. I own a handful of Android devices, mostly for development, but my primary phone is a Galaxy Nexus. Don't recommend the Galaxy Nexus though (at least not the Verizon Wireless LTE/CDMA Version [Toro]). Battery life is only 2-3 hours of screen time for it and 8-12 hours of total use. That's enough for me, not not for someone that uses their phone more heavily. The GSM variant is much better than that though and the S4 battery life will blow either one out of the water.
Honestly, it comes down to what you want, not what we can suggest. You don't give much in the way of details and background about yourself.
My biggest gripe with Apple devices is the closed-wall garden; I have no Apple devices at home (the only one I've owned was a 3rd gen. iPod Touch) . As a longtime Linux user, I don't see a lot of interoperability between the iPhone and Linux. The S4 also has a removable battery and an SD card slot, which is nice.
Also, thanks for the reminder about the Google Edition S4, might want to get that over the current bloatware-filled one.
If you are looking into development at all, Scala works well on Android if you prefer something other than Java. I only moved to using it the last couple of weeks (used C#/Mono and Java previously), but it's better than I thought it would be. I got it working without any external building or extra plugins in intellij idea as well (setup guide I put on github earlier today https://github.com/yareally/android-scala-intellij-no-sbt-pl...).
I'm stuck on Verizon for now and torn between keeping unlimited data and getting a newer device I won't hate. AOSP S4 is probably going to win me over in the end though and the nice AMOLED screen it has.