If it were me.... neither. Not sure what the rates are on the dates you're looking at, but Amantaka is much closer to the center of the action.
Amantaka is far from being among the best Aman properties in their portfolio, but La Residence Phuo Vhao was somewhat dark and depressing, and lacked the intimacy and privacy (similar to other Belmond properties in the region, including Governor's Residence in Yangon) and Rosewood is way too far - you won't get any sense of being in a very nice UNESCO town by staying there, IMO.