As far as I understand it, the difference is when we come to unobserve.
If we observeFieldScoped then unobserveFieldScoped we won't remove other observers (i.e. from another component) - but if we observeField then unobserveField we'll clear out every other observer too.
I tend to use the 'scoped' versions everywhere.
Watching this thread in case I'm wrong, that'd be good to know!