Personally, I would go via JFK, since you will get the AA Flagship lounge (and likely flagship lounge dining
) at Terminal 8. And of course a slightly shorter flight to MIA, which also may give you more opportunity for a lie-flat seat on the domestic flight, versus BOS, but this would probably be more luck than by design.
BOS - the only benefit will be if you absolutely want to be on a 747, but for me this is the poorest BA F option.