[Xperia Z Q&A, Help & Troubleshooting] plonkersaurus:
I went from a Galaxy Nexus to a Nexus 4, then had the same decision as you.

In terms of screen, the HTC One's has better viewing angles, no doubt about it, so does the Nexus 4 and Galaxy Nexus. In actual usage, on the latest firmware that improves blacks, it doesn't really matter that much. When actually looking at images, I compared the screens side by side and the Xperia Z seems to have a sharpness or vibrancy that the HTC One's doesnt. I really think the reviews overcook the view angle issue, it nearly put me off.

If you're used to the on screen buttons of AOSP, then I think the HTC one would be really irritating. That was a major factor for me, the quick swipe up from the middle launch of google now is also retained on the Xperia Z.

I found the HTC One difficult to hold, the edges seemed really sharp and hard, and the phone was quite slippery. At the same time the Xperia Z is also quite uncomfortable, but it doesn't feel like it is going to slide out of your hand. The power button placement is also much better on the Xperia Z.

The speaker is definitely much better on the HTC One, that said, the Nexus 4 is much better than the Galaxy Nexus, and the Xperia Z's speaker sounds better than the Nexus 4. So the Xperia Z isn't bad. I hold my phone with my left hand so it doesn't block the speaker, but if you hold your phone with your right hand and play music, then you will muffle the speaker.

I'm sure the HTC One is slightly faster due to newer processor but I don't think this matters that much, both will be fast.

I disliked sense 5 coming from AOSP, hated not being able to remove blink feed (find it really frustrating even though you can move it to a different home screen). The Sony customisations are actually an improvement on AOSP, things which frustrate me are fixed and also there are some other nice additions like location based wifi, bravia engine, wifi syncing with media go. The Sony Gallery called Albums is much much better than AOSP, the Walkman app is really good as is the Movies app. I never used built in apps before but I do here, I haven't installed MX Player.

The better camera in good light is undoubtedly the Xperia Z. I'm an amateur photographer (although street photographer so not much of a pixel peeper) and I'm impressed with the Xperia Z. Again the Nexus 4 is a step up from the Galaxy Nexus and the Xperia Z is much better. I haven't taken many pictures low light but I won't argue that the HTC One will provide better photos, the HTC also takes better videos and has Zoe which is cool (was one of the reasons the HTC One was attractive to me). Front camera is better on the Xperia Z than either the Galaxy Nexus or Nexus 4, the HTC One's is wider than the Z's if that matters to you.

The battery life I've been seeing has been pretty good, eg, if you're playing an mp4 film, I was seeing about 25% drop for 2 hours of watching the film. It's much better than the Nexus 4 which is a bit better than the Galaxy Nexus. Apparently the HTC One has a better battery life, I've never tested it though.

The Xperia Z has Micro SD slot which is great for music and films, I missed that owning the recent Nexus phones.

The last major point is the Xperia Z is water resistant, which is priceless, not because I want to take it under water but because it's nice having a life proof phone that you don't have to worry touching when you have wet hands or worried that rain will get into etc. It's a relaxing phone to own.

I only got the phone last weekend, the HTC One was the same price and I'm very happy with the phone and that I made the right choice for me.