They are pretty equivalent specs wise. The HTC One is a beautiful device, but it's really down to your own personal preference. There's things to factor in:
Do you need a removable battery? S4 has one, HTC one does not.
Aliminium body: The HTC One build is lovely and it feels great in your hand.The polycarbonate is more expensive on the S4 (afaik) but feels a bit plascicy and cheaper.
Do you need SD card storage? The S4 has this.
Do you like HTC sense or Touchwiz? Sense 5 is very pretty but I find the lack of customisation a bit restrictive, but you may love it.
The Samsung has a slightly bigger screen and is lighter.
The colours on the S4 are very bright and almost too saturated, but I like that, I'm a fan of AMOLED screens as I'm pretty colour blind.
Basically both are brilliant devices. I prefer my S4 over the HTC One, but I can see why it's getting such great feedback.
If you know anyone with the phones give them both a try for a few minutes.
Edit: Just to add you could always change your ROM on either device, there's a Sense 5 port in the works for the S4....