Disadvantage would be akin to that of the Euro, with forcing two economies into one currency pigeonhole. In your case, as your economies are so closely intertwined and have (at this great distance) similar political cultures, that wouldn't be much of an obstacle.
There'd also be the issue of deciding who/how to manage the new beastie, and that row would likely scupper any merger.
Adopting the $US would be a blunder, as your economies are not all that tightly tied to America's, and you'd run into a problems parallel to Argentina, tho nowhere near as severe.