I have two phones: an HTC One and a Lumia 920, so it's safe to call me a fan of both! A quick overview of the main points in comparing them:
Windows Phone is a great OS with an emphasis on glanceable information through the live tiles that let you get lots of quick little updates in a small amount of screen real estate. The OS in general really is very well designed in terms of letting you do your most common tasks as efficiently as possible. For example you have a 'Me' tile that will centralize all your major social network interactions, messaging, phone calls sent and received, and more. You can also pin tiles for your favorite people and it will centralize all your interactions with that person in one place. It's simple, efficient, and IMO a very fun OS. WP8 allowing you resize the tiles between small, medium, and large rather than just medium and large makes a huge difference in how customizable your screen feels.
Having said that, you have one long list of scrolling tiles, no folders and no customizable backgrounds. Some people feel really restricted by that. You swipe right to get a list of all apps sorted alphabetically. Once again no folders, but you can just jump to an app by choosing its starting letter very efficiently. It may sound very restrictive but there is a method to Microsoft's seeming madness because it is that restricted structure that lets them really orchestrate how things flow. Some people will feel boxed in, others really love how it all coordinates. The intention in general is to rely on their aggregating mechanisms rather than lots of individual apps. To put it another way: the OS is *task* centric, not *app* centric.
As others have said, app availability is still a significant issue. There are many excellent windows phone apps, but there are some heavy hitters missing. You can probably find a good app for most of your needs, but once again the OS is more about finding a good way to do your tasks and sticking with that, rather than having a huge array of apps all doing sort of the same thing. As before, some people will feel really restricted, and others will appreciate the simplicity.
For the phones themselves, (both of which I really like), here's my thoughts in no particular order:
- The lumia 920 is definitely a bulky phone, but I don't find it annoyingly so. While the HTC One clearly borrows from the iPhone design playbook, the Lumia is a quirky odd duck that really stands out. I really like its unusual design, it has very nice handfeel and is an example of a "plastic" phone that nevertheless feels about as indestructible as a brick. So, HTC One = Elegance; Lumia 920 = Inventively quirky
- Camera: The lumia 920 camera is still slightly better than the HTC One in low-light situations, though the One comes the closest to it of any other phone I've seen or heard of. The camera UI on the lumia is VERY well designed, and I like how camera-related apps are not separate, but instead treated as "lenses" that are usable from right within the camera app without having to exit out of the camera and go into the app. However, the One's Zoes, highlights & events, and camera options are much more powerful IMO. The Lumia 920 almost catches up if you download a few of the best Lenses. Overall I'd give the Lumia the edge on physical camera capabilities, but the One the edge on features/adjustments. For video I'd still say the Lumia is better because its OIS seems more capable. (It better be, because they sure bulked up the phone to squeeze it in there!)
- Sound: The Lumia has a good speaker and headphones sound good on it. It's Dolby processing is quite good and pretty equal to Beats in terms of how it makes music sound. I listen to music all day long so this is key to me, and both phones are well ahead of the pack. Having said that, I prefer sound on the HTC One. The Lumia sounded better than my prior HTC phones (even the Rezound), but with this latest version Beats seems noticeably better to me.
- Phone calls / reception: Both are excellent. The HTC gets better reception and calls sound better on it, but it seems a touch fussy in how I hold the phone when making a call in order to sound clear for the person on the other end of the line.
- Screen: The Lumia's is very good, and could almost be mistaken for an AMOLED screen because the colors are so vivid, and blacks are very deep. But, the screen on the HTC One is just the most superb one I've seen to date. It has colors that are strong w/o being over-saturated, and the resolution is just phenomenal.
To summarize, Windows Phone is a really smartly built OS. It'll feel restrictive at first, but if you do adapt to it's way of doing things you'll find lots of common daily tasks can be accomplished super efficiently and easily. Live tiles are fun, and Nokia is EXCELLENT about supplying some top notch apps for their customers for free.
I have a lot of affection for my Lumia 920, and think it is an amazing phone, but the HTC One is truly special, and is clearly the better overall package IMO. Having said that, I'd encourage you to go to a Microsoft store and have someone show you the ins and outs of the OS, it's different enough that just goofing with it a few minutes in the store won't do it justice. See if you fall in love with it: you just might. In that case, give the Lumia a shot, otherwise grab an HTC One and rest assured you're getting state of the art!
EDIT: I noticed the OP has been using WP 7.5, so lots of what I said is probably old news to you, but I'm hoping it'll be helpful to people less familiar with Windows Phone.