I have a Note 3 as my work phone and the M8 as my personal phone. They are really two completely different beasts, but I end up using the M8 much more than the Note for everyday use.
As far as the cameras go, don't get caught up in the specs, and you're better off dismissing the opinion of anyone that hasn't actually lived with and used the M8's camera . For overall everyday use, I much prefer the M8's camera over the Note. There are very few instances where I would get a better picture out of the Note.
Physically, the M8 is almost as tall as the Note, but much narrower. If you have at least average-size hands, it's entirely possible to use the M8 one-handed. The M8 beats the Note for build quality, fit-and-finish, materials, and overall design. Somehow, the Note manages to feel "cheap" next to the M8.
Of course the audio experience on the M8 blows away the Note.
Performance-wise, the M8 feels much more responsive and quicker than the Note. TouchWiz vs Sense is a matter of personal preference, but I think Sense is hands down the better UI.
For productivity purposes, it's going to be hard to replace the Note with anything else. The large screen format and S-Pen features are the main advantages the Note has over the M8.
I've been privileged to have the best of both worlds, so to speak. But if I had to choose one of the two, I'd go with the M8, and it wouldn't even be a very hard decision.