Anything you learn in International Style Waltz would be applicable in American Style, since the rules of footwork are (should be) the same. So, it wouldn't hurt to do International and switch over to American, to play around with shadow, underarms, side-by-side, etc.
You may just want to look at some good quality dancing at which point you can make up your mind, because the decision really is subjective in the end. Do you like people connected all the time, gliding as one, or do you like it when they mix up closed dancing with breaking apart and tricks?
I prefer American because I'm a fan of the Fred and Ginger movies, and prefer the freedom of movement, while continuing to train in the International technique that is required to excel in competitive American style.