Its most important to consider the features critical for the way you want to work.
That's the key point.
If I was definitely limited to 8 channels, the F8 would probably be a great choice, especially since it has 8 mic inputs as opposed to the 680's 6. It looks like a very impressive machine.
But...I'm not always limited to 8 channels. Even if I was most of the time, there are still times when I'm not, in which case the F8 would be, if not useless, far from ideal.
That's the name of the game when it comes to these digital recorders. While different machines can be superficially very similar, there can be make or break features on some for some people.