I just got my upgrade a couple days ago... Went for the HTC One though, but was considering between the S4 too.
I really like Samsung's Multiview, perfect for watching/listening to a YT live PS4 keynote whilst discussing it here in the browser.
The hover preview video is pretty handy too on the S4.
Still well happy with my One though, camera processing features are amazing, the removal and always smile features, so clever for a phone.
I also find Samsung's a bit phoney with their colours, too cartoon like, but have to admit the Multiview alone caused me days of weighing up pros and cons of each phone.
Closest can get to Multiview on my phone is with Halo. Which is all good for multitasking but it's not as good as Multiview.
Be good to see some Dolphin emu use on it