Always choose the path that will leave open the most options for you as you travel through life.
For example:
Check to see whether a Japanese veterinary degree/license will be respected in the US ("reciprocity" is a term that might help when hunting info).
We already know that a foreign degree/license can be used in Japan. And we know that an American degree/license will be good in America. So an American education leaves you a choice of location.
But what if a Japanese degree/license is not recognized in the US? Then in the event you decide to leave Japan (it happens, trust me) to go home, what are you going to do? There is also the possibility that you may not make it as a vet in Japan. The field is pretty much saturated, from what I understand. The possibility exists, then, that you could invest all that time and money into something that ends up a total bust (depending on license reciprocity).
You're still young yet, so you have plenty of time to get your ducks in a row. First off, look into that license reciprocity thing from the US side.