If you're feeling adventurous, I'd go ahead and reserve the Japanese meal. As others have mentioned, it's more authentic in style and ingredients than traditional "American" Japanese fare. (I still think the flavors are a little Westernized, but it's certainly a welcome change from the usual sticky sweet sauces in American restaurants.)

I've had the Western meals on the NRT flights, too; they're also good - more of the standard fare that you'll find on other international AA J flights.

Worst case, if you look at the menu and realise that the Japanese meal isn't quite for you, you can ask your FA to see if you could switch to a Western meal. Since the Japanese meals usually "sell out" first, my guess is that they'd be okay with the switch.