From what I gather:
The S4 is a beefier S3. It has the Snapdragon S600, 2GB of ram (same as last year's US S3), 4.99" full HD screen. The TouchWiz features in all honesty will probably be implemented to S3 by developers. In fact, it's safe to say that reviewing the phone based purely on hardware is the way to go.
S4 vs. HTC One:
Processor: Slight edge S4 due to 200mhz base speed advantage. This will most likely be overlooked when custom kernels drop. Verdict: Wash.
Ram: Both phones come equipped with 2GB of ram. Again, wash.
GPU: Same processor, same GPU. Adreno 320 which is currently the beast in 3dMark scores.
Screen: 4.7" LCD vs 4.99" (effectively 5") AMOLED. Both full HD so the PPI advantage goes to HTC with viewable screen estate going to the Samsung. Kind of a taster's choice if you ask me. Those who enjoy the vibrant/deep blacks of Samsung screens will undoubtedly enjoy the S4. Those who enjoy the bright whites and contrast ratio of LCD's will enjoy the One. The real choice to be made is whether you want a 5" screen or a 4.7" screen. If you think bigger is better, you will NOT be able to keep the One without thinking "I wish it was 5"".
Perks:
S4 has removable battery and a micro SD slot. A 16GB can turn into a 48GB with a $30 trip to Wal-Mart. HTC One has the clear advantage in build quality and feel. It just looks and feels better. (This is coming from someone who's picking up the S4 on the morning of the 24th) The HTC also has the better image quality camera if you're after low-light and better depth of focus.
The last real perk and this one, I'll probably get flamed for, but you HTC fans need to get over it and accept it: The S4 will undoubtedly have more developer support. There was practically an army of support for the S3 and that cult will follow through 2013. If you plan on flashing a custom rom, you will have as much support for the S4 (and options) as you would on a Nexus 4. HTC has become less and less popular on boards due to HTC's inability to meet mainstream consumer needs. Thinking about what engineers want in the phone vs finding middle ground between consumers and engineering finesse.
So in conclusion:
If you're happy with 4.7" of screen estate, less-than-top billing for development and okay with fixed storage/battery capacity, the HTC One is hands down the better device.
However, if you want 5" of screen estate, want to have the #1 supported device and you love having a microSD slot or being able to swap batteries, the S4 is for you.
Like your analysis & comparison a lot! Most likely I'm getting the white S4, though not immediately after release. I think the S4 is much more than just a S3-S; it's really much more than that. Every part of the phone (especially the screen) looks so beautiful to me!