HTC ONE, it's just so simple to use, and simplicity with a skinned UI is a good thing. I'm not a big fan of Samsung to begin with, so I'm biased towards HTC. But everything about the ONE is just so much better to me.
As far as gimmicks, the HTC ones that I can tell are the Boomsound, Zoes, Highlights, Blinkfeed, and the IR Blaster. They actually come in handy and I use them all the time. As for samsung, they packed so many gimmicky features that they just get lost in a sea of gestures and swipes and tilting... It's complicated, to me at least.
-Luis