[General Questions and Answers] Planterz:
The 5.1" screen on the S5 is obviously significantly bigger than the 4.7" on your M7, but the phone itself is only marginally bigger (thinner, actually), and a mere 2 grams heavier. If you want something more in with the screen on the M7, check out the Galaxy Alpha. Still 4.7" screen, but the phone is significantly more svelte. The downside is there's very little in the way of development. Don't forget about the S4 either. It's very near in dimensions to your M7 and still a competent device (a bit faster than your M7).

The other obvious advantage to the Samsungs is the removable battery. You're about to spend hundreds of quid on a new phone because of the dying battery, but this is a cheap fix for the Samsungs - just buy a new battery off eBay or Amazon. The HTC One M series might have some fancy construction, but this makes it damn near impossible for the average Joe to fix. Your phone is likely out of warranty by now, but you might still be able to send it in to a service center for a battery replacement. No idea what it would cost though.

As for TouchWiz being awful...that's a matter of opinion of course, but it's one I share. Frankly, the S5 is particularly awful. The settings menus are where it's the worst. Deep menu trees, too many tabs, and it's completely unintuitive. For example, to turn off S-Voice (Samsung's needless redundant version of Google Now) from activating from double tapping the Home button, there's no settings in the Settings. You have to load the app, tap your way through the User Agreement and tutorial, and then finding the settings within the app to disable it from there. (The reason to disable this function is because when you hit Home it's waiting to see if you double tap it or not, causing a bit of lag/delay).

That isn't to say that TouchWiz is impossible, or even bad, really, but it's far from the simple AOSP base, or the more streamlined HTC or Sony UIs. The real problem is that there's just so many settings, and so many redundancies and/or features that you'll never use. So when you first get the phone, you've got to find them all and turn them off so you don't have 700 processes running and slowing the phone down, and it's an arduous trial for a first-time user. Nothing that an intelligent person such as yourself couldn't figure out, but to a casual user it can be off-putting, and even for techies like us it can be tedious and obnoxious.

I have 2 Samsungs, one a Galaxy Light (a cheap phone for the NA market) and an S4. The Light, being a budget device, has far fewer features, but it still confuses me sometimes, despite having owned it for over a year. My familiarity though helped me learn my way around the S4, which has a similar (just more feature-filled) interface. This familiarity doesn't help me much with the S5 though. The few times I've tinkered with an S5, my reaction was "Gah, WTF?" You expect similarity, yet it's so different.

I'm not telling you this to dissuade you from Samsung, I'm just trying to give you perspective on why it's considered by many to be awful. Those that do usually do so in comparison to other UIs. It strays greatly from the simplicity of AOSP, so it'll never be looked favorably upon by those of us who prefer stock Android or custom ROMs like CyanogenMod. Of course, HTC Sense strays greatly from AOSP too, but in different directions than TouchWiz. Which means you'll probably find TouchWiz a lot like crossing the pond and driving on the other side of the street (or like when the Grizzwalds went to London and couldn't figure out the roundabout).

Despite my dislike for the TouchWiz settings system, I like my S4. It doesn't stutter or lag or struggle with intense 3D games. Although there are plenty of AOSP custom ROMs for it (like LiquidSmooth, my favorite), I keep it on TouchWiz because I actually do like some of Samsung's features (plus I have LS on my OnePlus One and Nexus 4, so depending on my needs/mood, I just swap the SIM).

I guess what I'm getting at in my long-winded post is that you shouldn't let the negative opinions of TouchWiz stop you from looking at Samsung products. Once you figure out how to turn off all the stuff you don't need and that slows your device down, they're perfectly useable and you'll probably find some features you do like.

As for other alternatives, it might be worth it to actually go back a step or two. The Moto X 2013 and Nexus 4 are still perfectly competent devices, both with 4.7" screens. The batteries aren't hot-swappable on these, but replacing them isn't impossible like it is on the HTC's. There's also the LG G2. The 5.2" screen is significantly bigger than the 4.7" screen on your M7, but the phone itself is only marginally bigger, and it's actually smaller than the Galaxy S5. I guess it depends on whether it's actually the screen's size you're concerned about, or the phone overall. The One M series has the worst screen-to-body ratio this side of an iPhone. The M7 has significant side bezels, lots of unused black space between the screen and speakers, which themselves add to the height of the phone. Compare the M7 to the Moto X 2013 and it's hard to believe the screens are identical sizes. The reason I mention these phones is that, as well as still being good devices, they're dirt cheap now. Heck, you could just about buy all three for what a new S5 costs. Moto X for when you want a smaller phone, G2 for when you want a bigger one, and use the Nexus 4 to learn about and play with custom ROMs and kernels.:D

I can't comment on the durability of the Z3 Compact. Put a tempered glass protector on the back, and use a decent case, and I'd be surprised if your phone cracked for no apparent reason. The battery isn't replaceable (no idea on repairability), but I will say that it has phenomenal battery life, which means the battery will last longer. Not just in the obvious sense, but charging/discharging is what degrades lithium batteries over time, so longer daily life means less charging, which means a longer lifespan over the years.