My take.....Galaxy S3 vs. HTC One M8
Putney, I came from a S3, and I'll give you my take.
The Galaxy 3 is a really great phone with many ways to tweak it. The HTC One M8 is still somewhat new, so you don't see as many tweaks you can do to the HTC as you could the S3. I haven't tried any of the few ROMs that are available for the HTC because I think the stock ROM runs really nice. It's not only smooth, but with the phone's faster processor, you'll notice it moves along faster than the S3. This is not to say these ROMs being developed aren't worth checking out. Obviously with more time, developers will come up with more and more options!
With the new Kit Kat that is installed on the HTC, you will have to install a workaround so you can use an Ext. Card as a storage device, because Kit Kat has enacted safeguards to only allow access to certain programs. I got mine to work, so it is possible to do. Of course you'll have to go through other steps to allow you to "free up' your HTC.
The sound is definitely better on the HTC with two speakers! The screen is bigger and looks great.
A big negative I've found, is the fact you can't access the battery. I liked the fact with the S3, you could change to a bigger battery size so your phone would last longer. My S3, with my new upgraded battery, lasted up to a day and a half. I don't think the HTC can even make it 24hrs. The HTC has a feature that you'll have to patch in yourself, or it comes with the new update, called "Extreme Power Savings Mode" that is supposed to save the battery. I just installed it yesterday, so I haven't used it enough to report on it.
Generally speaking, though, I'm glad that I went to the HTC. It obviously has other great features that I haven't touched on and the reviews I've read have been very positive. Never thought I'd give up my Samsung, but it looks like I made a good choice!
Just my 2 cents!