I'm in the same boat. I'm currently using an iPhone 4S and I'm torn between the GS4 and the One. This will be my first Android phone after being on iOS for the last 6yrs.
The big thing for me is the camera. The 4S takes great shots (of course lighting has a lot to do with it too....and maybe an app or 3
) Anyhow, I'm leaning towards the GS4 because of the camera. I know the One takes excellent low light shots, but from what I've seen for brightly lit or day light shots, the One lacks the detail when the pictures are viewed large. The GS4 retains that detail in it's shots. Sure, again from what I've seen, the GS4's pix may be lacking a little in the color saturation dept., but that's nothing an app can fix.
So for now, since I can't upgrade for a month, I'll continue to do my homework.