It's really a matter of opinion.
However there are really some pros and cons between the two.
Galaxy S4:
Pros:
- Best performing smartphone on the market.
- Can get repaired by yourself with no trouble.
- One of the best cameras in a smartphone on the market.
- All the features you can imagine on a smartphone, there are apps for everything when you get the phone.
- Expandable storage.
- Battery can be changed.
- AMOLED display produces fantastic colors.
Cons:
- Built quality is not up to par with similar priced smartphones, due to the expandable storage and changable battery.
- Has not much storage when you get it, due to the preinstalled apps from Samsung and you're limited to a 16 GB version.
- Design is neither up to par with similar priced smartphones (subjective matter, I really like the design)
- Bad low light pictures
HTC One:
Pros:
- Built quality is fantastic.
- Probably the most sexiest phone you've ever seen (subjective matter again, I still like the Galaxy S4, but there is no denying that HTC One is better looking)
- Best audio in a smartphone atm.
- Has lots of storage when you get it, compared to Galaxy S4 due to less preinstalled software. Also feature more storage options to buy.
- Good low light pictures
Cons:
Unrepairable by yourself.
Can't expand storage.
Can't change battery.
I MYSELF:
Was also in doubt which one I should buy. But I chose the Galaxy S4. I needed a good camera, I like expandable storage for my movies, I would like to have the options to repair my own phone to save money if I drop it and I wanted the beautiful AMOLED display as well.
Again as mentioned at first, it's a matter of opinion. Both smartphones are really great, you can't go wrong with either. And that is actually the problem, it makes it hard to decide.