So I got an iPhone 4 from a friend today to a/b it with my Htc One. At first wanted to go with the iphone 4 coz I heard it was the best sounding iPhone using it's headphone out. After comparing it side by side with the htc, the htc is superior to the iphone 4 in instrument separation, vocals, bass, and soundstage minus the fact that the iphone 4 drives my hd 558 a bit louder than the htc one.
This is just my own brief experience and ofcourse ymmv.
I was using poweramp(all stock settings) on the one on android 4.2.2 whereas stock player on iphone 4 running 6.1.13 firmware.
So the things my ear was able to pick is the vocals, the htc one makes you feel like your sitting in front rows and the vocals are more lively and a bit clearer than on the ip4. The instruments have better layering and separation too on the one. As for the bass I would describe it more precise and a bit more punch but not bloated or anything on the one whereas on the iphone 4 it was a bit lacking and I'm not a basshead by any means.
I did the a/b volume matched(around 14/16 on the iphone 4 and volume was on max on the one).
These difference were subtle when I used the HD 558 but it was more apparent when I used the phillips she 3580. These two are the only equipments i have at the moment.