Avios F availability from MIA is almost non-existent, so I wouldn't book with that in mind. I currently have a booking in CW from MIA in a couple of weeks that is F9 A9 I9 E9 G9 O9 with just one seat currently pre-selected in F, and there is still no Z class available.
As for AA Biz vs BA B744 UD, you seem like you know the seat and airlines well enough to be able to make that decision based on your personal preference. Apart from the flight I just mentioned which was booked on BA due to the late timing, I would take AA, their main drawbacks of long noisy service, brightly lit cabins etc are more critical on the super-short overnights from the US Northeast but at least MIA is long enough that you should get some sleep anyway despite their attempts to stop you. If you are able to sleep fine in the CW seat and want the lights off quiet of the UD then go with BA.