Thank you very much Fiendish Astronaut for an awesome, informative reply to your awesome experiences there.
I will do at least a full day and perhaps more with my time in Siem Reap (but I will have business to take care of also during my down time there before heading to Singapore and back to Australia and New Zealand).
I am more excited now about AK and the other temples.
Thanks again.
PS. This is 3rd trip to Vietnam, 2nd to Hanoi but 1st to Halong Bay and I'm taking a Bhaya cruise for 3 days/2 nights to get the full experience.