Hi Eldy3... I understand your predicament, I decided to jump from an iPhone to android and couldn't decide which phone to buy either so I bought the S4 and the One.
Let me start by saying that it is the S4 that has ended up on Ebay.
The pros of the One for me are the magnificent screen, the dual front facing stereo speakers, the design and feel of the unit, and the software that gets you up and running.
The software allows you to import everything, including SMS messages, from an iphone or another android phone to HTC Sync and away you go. I love this, especially as a user switching from an iPhone, it's great to have software that makes this switch so easy.
The dual front facing stereo speakers are WONDERFUL. Shortly after getting the One, I remember thinking 'why aren't all phones made this way, it's such a no brainer!'. In comparison, when the S4 is laying on a table face up, the speaker is covered and the ring volume is greatly reduced. Enter the One's victory. Also, how many times have you had to cup your hands over the rear or side facing speaking to hear audio on your phone? The One does away with that all together.
The built in keyboard I find very easy to use because all of the numbers, letters and even the most commonly used special characters are on the keys right there, all you need to do is press and hold. There's no need to hit a button to take you to the symbols and then another one to take you to the numbers, it's all right there. You can also press and hold for smileys and voice recognition. The only DOWN side I've notice so far is that the keyboard does not enter a space after a word that you accept from the 'next word prediction' unless you then select another word from the prediction. So that means that if you type 'what', it will predict the next word to be something like 'does'. If you select 'what' from the prediction after just typing WH and then start typing another word manually, it will not enter a space. Spaces are only automatically entered when selecting consecutive words from the predictions. Having said that, it's very easy to get used to, and at least it has predictions where the iphone doesn't!
Of course having no SD slot or exchangeable battery means nothing if you've come from an iphone like myself because you'd be used to it. Going from a 64gb iphone to a 32gb One was a squeeze but it just meant that for now I have to reduce my photo's from the last 30 years to the last 3
All of my music fit, which was only about 11gb.
One more thing which astounded me coming from an iphone, was that the One can't make video calls by default; it needs something like Skype. This was a big thing for me when I realised it and was one of the reasons I rushed out to get the S4 and give it a try. After playing around with it and realising that most of my friends have iphones for now and I can't video call them anyway, I got over it.
I also find the automatic screen brightness to work much better on the One than on the S4. The screen on the One is gorgeous, but I found the screen on the S4 to be nothing stellar. Bear in mind I came from an iPhone 5. I'm not sure why given their all great ppi's but I find the One screen nicer to look at than the S4.
I hope I have helped in some small way!