I was trying to make the same decision myself and decided to buy another HTC EVO 3D (my current phone) instead. As great as both phones are, neither warranted upgrading from my current Android at the $280+ (early upgrade on my plan) price point. So for approximately $155 CDN I bought a brand new EVO 3D.
I personally prefer the HTC One. It is a better quality product all-in-all. The construction is far superior, with a much better look & feel. The phone is on par with the iPhone 5 in terms of sleek looks. The Samsung S4 simply looks cheap, its made entirely of plastic with dated round edges... blah
If you're going by numbers, the S4 should in theory be a better phone. However, real world tests seem to be saying the opposite. The One has been widely reviewed as feeling more responsive and less laggy than the S4.
Some things to consider:
Performance: The Samsung Galaxy S4 has more CPU cores than the HTC One - however, more cores means lower frequency per core - which equates to lower performance in applications not optimized to utilize the extra cores. This is only an issue during this transitional period of duocore to hexacore and octacore configurations.
Display: The Samsung Galaxy S4 has a larger screen than the HTC One - however despite being .3 inches larger, the Samsung's PPI (pixel per inch) resolution is lower than the S4, resulting in less clarity and color definition.
Camera: The Samsung Galaxy S4 has a 13 megapixel camera, the HTC One doesn't - HTC has yet to publish the actual resolution of their 'ultrapixel' camera in any publications... The camera is effectly 4 megapixels. However, the term 'ultrapixel' is used to identify HTC's new optical CMOS sensor, which is effectively 4X larger than the CMOS used in any other smart phone camera. The sensor is so large it compares to entry level digital SLR cameras. It is capable of capturing substantially more light and detail than its 8+ megapixel counterparts. In all, it effectively captures better images than the S4.
Interface: For those who have never had to privilege to use HTC's Android UI 'HTC Sense', please do yourselves a favor. It is by far the cleanest, easiest to you use, Android UI on the market. The amount of well-thought out and high quality widgets is amazing. HTC has taken every great feature of the Android OS and beautified it into a simple and intuitive interface called Sense. I've tried both the Samsung UI and HTC's, the difference is night and day IMHO.