Both airports are way out of town, so I would say that the time taken to arrive either at Hanoi or Siem Reap is the same.

The question to ask is the flight arrival times, as this can kill a day if they are unhelpful.

Both citys are HOT at the moment. I have travel last week from Hanoi to Siem Reap and one is humid hot and the other is well humid hot !

Choice of hotel also is a factor

TBH I would say Hanoi as there are lots of "things" to do that do not require much walking. - The water pupet show ? - Siem Rep is basicaly temples - and unless they want to spend a day at the back of a hot Tut Tut rule that out

It depends on many factors. This is my 2c worth