The Belmond is a travesty from what we've heard from numerous friends and colleagues who've stayed there. Don't do it.
The Amansara is spectacular, though it obviously has a 60s mod design that some seem to dislike. We loved it--and the service and F&B were true Aman. While it is more expensive, remember that your private guides and transport for all your daily excursions are included to see the many Angkor sites and temples--and their guides are among the very best in town. (I do know the PH also used the same group of guides.)
Finally, Amansara allowed us to modify our private excursions to whenever we wanted--period. That's proper luxury and proper service. You pay for it, but it gets delivered. We left later to avoid crowds and did all sorts of things that we wanted, when we wanted. You can't do that as easily at the lower price points.