Even without making any analyses on the "data" (for lack of a better word) you've provided, and by just purely going with how you have presented the info and what you sound like you truly want, your heart is telling you B. Furthermore, you have the data to back up the decision, so it's not like you're making an emotional, knee jerk decision. Go with B. Plus, how amazing that you have such a supportive advisor at A who if you play your cards right you can still keep in your corner for years to come (hello, postdoc!). And finally, in 5 years, 10 years, no matter which program you choose, you will be where you want to be and you will be happy, because your situation will be what you make of it and you will make the best of it. It is so hard when you are in the thick of it to be able to step back and take the plunge to make the final decision (I have DEFINITELY been there!), but B is an awesome choice and a great opportunity for 5ish years.
...and then it will all be moot because of course the second you make your painstaking decision, you'll get the Fulbright and go to B anyway and all will be right in the world!