Hmmm. I'd have a lot to say to a newbie, but I'd bet you'd know pretty much everything I'd say about it.
Regardless, I'd say that Ni-doms tend to be more layered and naturally decisive, with their convictions being difficult to elucidate (make transparent) because they really don't have any specific reasons for them that they can easily pull up and point to.
Hmmm.....in terms of believing or agreeing, I'd say Fi believes in personal values. Ni believes in ideas and theories; even when it's one about feelings.. I...would say Fi is more emotional than Ni.
Say, if we're trying to convince Fi and Ni dominant users..... clicking life stories is an instant go-go to Fi users. Ni, on the other hand, a brilliant idea, if it clicked, is..more or less agreeable to Ni, even when the idea itself is not so agreeable.
In terms of stubbornness (which, all functions can exhibit); Fi focuses on feelings and needs of oneself, while Ni focuses on their ideas.
Fi : "I want to be a president, I want to be a president, I MUST BE A PRESIDENT OR ELSE."
Ni : "Being the president is the best job for me. Being the president is the best job for me. Nothing would do!"
Overusing, Fi becomes selfish, while Ni becomes closed.
In shadow, Fi >> Fe made one into blaming the world / going totally aggressive, while Ni >> Ne made one lost in possibilities
[MENTION=1926]April[/MENTION]; I wouldn't say so; rather Ni is a quick-to-draw magic power that, more often than we could like to believe, fails to hit.
Fi, on the other hand, is a slow-to-draw-and-cast magic spells that, once hit, REALLY hits.