I honestly really like both phones. There will always be advantages to one phone over the other, but honestly (unless you are a complete pessimist) both phones should be able to keep you happy for around 1-1.5 years minimum.
The thing that I want to understand is the HTC One Capacitive Buttons. You have home and back... but where is menu? Menu is probably one of the most essential buttons in android. Hopefully (this is what I would do) hitting home once would bring up menu, and double tap will go home. Something along those lines. Nothing I am really worried about anyways (AOKP will fix it haha)
I do not really think UI is something to get in a heated debate about... once custom firmware hits you can put whatever you want on. Personally I am going to rock stock AOKP on any phone I get.
Same goes for storage.... Sure XZ has a micro SD slot, but there aren't any micro SDXC cards bigger than 64gb (and might not be for a while), so really the difference between the HTC One and XZ is 64GB vs 80GB at maximum.
The only real criticism I have for Sony is the delayed North American release for the Z and especially the ZL. HTC revealed the phone today and will release in a Month. Xperia ZL was revealed in mid January and wont come out till April. There is no point to reveal an amazing new phone and then release it once it is old news. Shame on Sony for that.
Both phones are awesome, just go for the one you prefer.
Battery life is something I am finding more and more confusing with Android phones. How is it that the HTC Butterfly gets a better battery life test during video streaming than the Xperia Z by over an hour, yet has a 13% smaller battery, same processor,same GPU, and same screen resolution. This seems completely counter-intuitive.
Now for my personal opinion. I prefer the outer design and features of the ZL over the HTC One (I like the idea of as little bezel as possible), however I prefer the One screen and speakers. Not going to make any decisions till the Galaxy S4 comes out.