Well steak tartare, (if done right, oldschool traditional)is finely chopped filet, served with an egg and some onions and stuff with it. And you can then prepare it on your plate.
Americain prepare, is a close cousin of that. But where the Steak tartare has already been prepared for you and it's chopped.
Americain Prepare would be made would really be made with Filet Americain (minced filet, not chopped), and it wold be prepared by the cook or butcher..
And because of the mincing, and processing, it becomes a more fine spreading paste kind of thing.. That's one way we eat it here, we spread it on our baguette
But many restaurants will serve Steak Tartare with a mince, and then the difference becomes less pronounced, especially with a more old school Americain Prepare, where the meat has processed into a paste like consistency..