Here are my 2 cents.
The S4 has managed to convince people that it has the ultimate blazing fast superphone, and it does to an extent. But, the so-called octacore is just a gimmick to get people to believe that they will actually be getting a smartphone powered by 8 supposed cores(which is total overkill and extremely unnecessary, also its technically only 4 cores). The idea of having 8 cores is one of the main reasons why people will choose the GS4 over the HTC One, when in reality the One has more than enough processing power, and anything above that would be overkill. On another note, the S4's screen is stunning, but unrealistic in the sense that AMOLED screens over-saturate colors. Another plus for the screen is its high brightness level, which not everyone cares about. But the One packs more realistic colors and provides better viewing angles. The PPI difference between the two phones will literally be unnoticeable at this high of a number, unless you like using your phone 2mm away from your eyes and frequently scrape your cornea in an attempt to see the actual pixels. Among the gimmicky new features of the S4, there are actually some useful ones. But, the constant use of the front-facing camera in a few of these features might pose a threat to battery life. The rest of the new features are basically features found on other phones, but they seem new to most people since they are sported on a major flagship phone. As for battery, I've seen mixed results from the HTC One battery, but have yet to hear anything about the S4's slightly larger battery. But what the true game changer should be, when you think logically, is the removable battery and the SD card support found on the S4. For people like me, this doesn't matter, but for others it could determine which device they buy. In the camera field I beleive the One has a better overall camera, some of the comparisons seen online seem to use the worst possible settings for the camera, making it seem quite bad.
In conclusion, the decision comes down to having a popular piece of plastic that performs exceptionally, or a sexy piece of metal that basically performs just as exceptionally(of course with a better camera).
Also; dual front-facing speakers. I don't really need to explain that though