I have a book on the P-61 and it stated that a U.S. nightfighter squadron was going to receive the Mosquito because of negative reports on the P-61. A contest was held between the two aircraft and it was shown that the P-61 was more maneuverable.
I also believe that they would prefer U.S. aircraft. With a British aircraft, you would be flying on the wrong side of the sky!
The Beau was better? How can U say that? Look at the vision fields of the crew members in the P-61 versus the Beau! The Radar systems! And the Beau did not the the option of a belly attack...wheras the top turret of the P-51 gave that option, though it wasn't AFAIK actually used in War Two.