Apparently nothing, other than phonetically. If two words begin with F, then tra is used and vica versa. Check out http://italian.about.com/od/grammar/a/italian-preposition-tra.htm
It's also Canadian to say "amongst", and I use British English and American English interchangeably all the time ( as do most Canadians). I hadn't noticed yet anyway, a preference on Duolingo's part for one or the other. What I have noticed is that Duolingo will often reject a correct translation (from English to Italian), and there's no explanation for that given.
If you want to nudge American English toward your preference, then I fully respect that. (I do the same with other words.)
However, you should be aware that as it currently stands (as of March 2017), "amongst" and "whilst" are not usually considered a part of normal American English, but perhaps you and other like-minded people will succeed in changing that in the future. Good luck!
"tra di noi" should be accepted. "tra is the more common form; proceeding personal pronouns it is often followed by the preposition di: tra di noi." --http://italian.about.com/od/grammar/a/italian-preposition-tra.htm
Say I was to translate this as between rather than among, can anyone tell me if it can be used both literally and figuratively? For example:
"where are seats D5-12?" "they are between us."
"my love! Why do you reject my proposal so?! Do you not care for me as I do for you?" "it is not I but my family that reject you! They are between us!"
Okay, maybe not my best examples ever but you get the idea. Can fra be figurative?
https://cdn.discordapp.com/attachments/431549337370230797/431549387336843274/What-Was-I-to-do.png Also there was no specific way to report this so I hope this works
They can be used interchangeably, so really whichever you want. However, some dislike saying the same same syllable twice in a row, which makes you change between Fra and Tra. In the sentence "between Frassinoro and Florence", you could say either: "Fra Frassinoro e Firenze", or "Tra Frassinoro e Firenze". Both are correct, but some find it awkward saying "Fra Fra...", and prefer saying Tra before Frassinoro.