I'm gonna go with Apple because I flat out don't like any of Samsung's phones. Most of them are ugly, especially the entire Galaxy series (I kinda like the 1st Galaxy, though).
I hate the plastic, cheap, light feel. Samsung's Android UI (the name escapes me) is pretty good, though. I'll give them that much. And the S4 has a bunch of great new stuff, like how you can use the touch screen without actually touching it. And, of course, the eye detection features. I like all that.
But the ONE thing that REALLY gets me about the S4 is the megapixel count on the camera. People call out Apple for the iPhone being a marketing gimmick when the S4 has a 7000 MP camera. If you know anything about cameras, you'd know MP count doesn't mean shit, really.
My 4S has been my trusty pal for a bit over a year. Like Wade said, there's nothing on Android I'd really want. I have mine jailbroken. Someone brought up how jailbreaking voids your warrenty. If you break your phone, then just restore your phone in iTunes and you won't be jailbroken.
My next phone's probably going to be an HTC One, though. People who argue that iOS has been the same for so long are pretty much spot on. I've only lightly treaded on Android, so I wanna see what it's really like.
The HTC One is a beautiful looking phone. The Sense UI is a bit "meh", but I imagine I'll get used to it. The thing I'm afraid of is getting the One and missing my two year old 4S.
Just have to wait for this fucker to get on Verizon.
I considered the Lumia 928, but the 920 is a bit better from what I hear, so fuck that. I'd rather wait for a 1020.
Also anyone who says DROIDS (do not mistake that for ANdroid) are the best, you're fuckin wrong. Exept for MAYBE the Razer Maxx because POSSIBLY Motorola cleaned up MotoBlur and it has a legitimately sweet battery, the Droid line is SHIT, MotoBlur is SHIT, and everything about it is SHIT. Droid 1-3 are the only phones I fucking hate.