Should I update my One M7 or M8 to 4.4.3?

This got a very simple answer: YES! Just DO it.

There is not much to consider here. Esp since it contains security fixes that patches all known OpenSSL problems and the FakeID issue.

If you heard that someone got a problem after they’ve installed the update, that’s their phone and not how the update in general is. Some phones got issues, read more about that HERE.

On One M8 it’s so worth it, since you get the HTC File Manager that lets you create folders on external storage. AND: lets you copy things between internal and external, this wasn’t possible before due to Android 4.4 security.

And: if there’s a problem with the update, it will get an update. As simple as that.

Oh, it’s worth to wipe the cache partition as well. More on that HERE.