[One (M8) General] Hlorri:
It was the reverse for me too. I've had the 64GB version of the original HTC One (m7) from AT&T for a year, then briefly upgraded to the m8 once it came out this spring. I quickly returned it however, and have since gotten a Z2 which is now my primary phone.

Between the 3 devices, each one has some advantages over the others. These are:

HTC One (m7) over HTC One (m8)

Optical Image Stabilization.

micro-SIM

(up to) 64GB of storage built-in. This is both a pro and a con:

One problem with external µSD cards is that starting with Android 4.4, applications can no longer write to folders outside of its own data folder, so applications like BitTorrent Sync will not be able to work. This is not an issue on internal storage.

On the other hand, with the M8 or the Z2, you can now use 128GB cards for more than twice the storage; and with the proper fix (NetApp SDFix from the Play store) you can eliminate the previous issue

HTC One (m7 and m8) over Xperia Z2

Zoe! If you capture a lot of moving objects (overactive kids etc), this is invaluable to help you capture the "best" (and most blur-free) shot.

Better email reader, better email + calendar widgets/shortcuts. I especially like the mailbox-specific "shortcut" icons on the desktop with an unread mail counter, bringing you straight to that account's inbox (or any other folder), as well as the 1x4 calendar widget. Corresponding icons/widgets on the Sony Xperia's are more klunky. (The minimum size for the native calendar widget is 2x4, and 3rd party widgets don't work so well).

More active developer community -- more hacks, more custom ROMs. In particular I found the tweak to disable Exchange security on the m7 email reader quite useful - there does not seem to be a similar hack available for the Z2. (Because of this and the previous point, on the Z2, I'm now using the "Nine" mail reader for my work mail; this allows "application level" security at least).

Blinkfeed. Sony "Socialife" falls a bit short. I have not tried Flipboard (Don't like that I have to register for that, too) - still looking for a good news/social media aggregator.

More ergonomic, especially for single handed use.

More accessible µUSB/charging port. With the Z2, you either have to deal with the side flaps covering the ports, or else use the included magnetic charging dock and/or a Z2-specific charging cradle or receiver for your car. Neither cradle works if your Z2 is dressed up in a case.

I didn't much care for the Duo camera. This was the main reason I got the m8 in the first place, especially for the "background blur" feature. It fell quite short of my expectations, and was not really worth the loss of the OIS from the m7. Hence, returned.

Z2 over HTC One (m7 and m8)

Better camera. Not so much because of the extra megapixels (images from the Z2 are a bit noisy at the pixel level, and don't really have a lot more detail than the HTC one), but because of better color balance, less susceptibility to overexposure, blooming and white-outs. HTC do have a point with regards to the theory behind their "ultra-pixels" - if only that theory were applied well in practice.

Bigger screen - still extraordinary crisp!

Noise cancelling headphones included. This is great when on a noisy train (as I am to/from work every day) -- I often wear them even if I'm not listening to anything.

Better built-in keyboard (though I install "Swype" from the App store in any case).

Speakers are great on all these devices. That is, the original HTC One's are a bit weaker after a firmware upgrade sometime back, but with custom firmware you can get the original sound volumes reinstated; hardware-wise it has the loudest and cleanest speakers of the 3.

Hope This Helps...