[Galaxy S 4 General] BarryH_GEG:
When you compare the two there's not a lot that's unique going for the One except for it's beautiful all-aluminum design and dual speakers. The fact that it's the same physical size (while being thicker and heavier) as the SGS4 with last year's 4.7" display size isn't going to help it. Here's a quick run down comparing the two. It was the answer to "why would anyone buy the SGS4 over the One" from thier forum.

Larger 5" SAMOLED display with vibrant colors

Thinner and lighter with other dimensions being near equal

13MP camera (more detail)

Better battery life, bigger and removable battery

Replaceable back cover if its damaged

Expandable storage

Gorilla Glass 3 (versus 2)

Android 4.2.2 (vs. 4.1.2, at least for now)

Dual Camera (insert yourself in to photos you take)

Drama Shot (a simpler Zoe)

Sound and Shot to record voice comments on still photos

Story Album to create and send printed photo albums

Share Music to turn multiple SGS4's in to surround sound

Group Play to share content across multiple devices without the need for a Wi-FI connection

S-Translator integrated in to multiple apps like e-mail and the stock browser

Smart Pause/Smart Scroll to use your eyes to control the device

All the existing motion and voice controls Samsung provides in TW

Air View to expand content by hovering your finger over it

A display you can use with gloves on

S Voice Drive which maximizes the size of what's displayed and allows motion and voice to control things you use when driving

Optical Reader integrated in to stock apps to import data via the camera

S Health to track weight, blood pressure, exercise, etc.

I'll take a stab at the One's advantages over the SGS4. For the sake of discussion assume the SGS4 being compared against is an LTE version running S-600 too. Things like LCD vs. AMOLED and Sense vs. TW are obviously based on personal preference.

Beautiful all-aluminum unibody design

LCD display with bright realistic colors

Sense’s more aesthetically pleasing appearance

UltraPixels (better low light performance)

Dual front facing speakers

Zoe

BlinkFeed

It’s not made by Samsung (for the haters )

Other than Zoe and BlinkFeed Sense 5 is carry over from earlier versions. Even LG's doing a lot more with s/w than HTC. The lack of s/w features, whether they are real or gimmicks, is going to hurt the One with the masses who like "more" rather than less; especially at the same price.

LG already offers several special apps on its Android smartphones, like QSlide for viewing two apps at once on a device's display and QRemote for controlling home entertainment gear. Today LG expanded is software offerings with another new feature dubbed "Smart Video." According to LG, Smart Video takes advantage of eye recognition technology to track a user's eye movements while he or she is watching a video and, when it detects that user isn't looking at the device's display, it will pause the clip automatically. Playback will resume when the device detects that the user is once again looking at the screen. LG says that Smart Video will be rolled out soon as part of a Value Pack update. In addition to Smart Video, the Value Pack will include a Dual Camera feature that will take photos using the front and back cameras simultaneously, Magic Remote Keypad and Text Keypad additions to the QRemote app, Video Pause/Resume that will allow users to stop and start while capturing a video to create one continuous clip, and a set of color emoticons. The Smart LED Lighting surrounding the LG Optimus G Pro's physical home button will also be updated to allow users to assign colors to specific contacts.​