I realize I left out some information...we also are interestied in a Halong Bay day trip. We need to go in and out of Hanoi, so in Summary what where would you go if you had to fly into and out of Hanoi - Day 1 being arriving in Hanoi, 2-3 free days in Hue/Sapa/Halong Bay/??? , then 3 days back in Hanoi.

We are open to flying into Hanoi only one way if there is a cheaper direct flight back to HKG.

Not sure from your post exactly how much time you are intending spending in Vietnam but it seems to be around 5 days. If so, I would say that you could easily use that time to great effect just in Hanoi. If you do want to include a second loaction then I feel tha sapa and Hue are just too far (I did not really take to Halong bay but that is just my subjective view). I would be inclined to do an overnight trip to the Mai Chau valley. Very beautiful scenery, wonderful people and a really peaceful place to wander around teh countryside. We stayed with a local family in a homestay but I believe there is now a upmarket lodge in the vicinity if you fancy more luxury. All easily arrange with one of teh tour operators in the old quarter.

We will probably arrive in Hanoi on a Wednesday and need to be back there Saturday - Monday / Tuesday. I don't want to spend 5 days in Hanoi.. we could have 2 full days to go elsewhere. How long is needed to get to Sapa?

Thanks for the information about Mai Chau. The Mai Chau lodge offers packages that include the 3 hour each way transfer from Hanoi. This might fit well into our itinerary.
If you had the time and had to choose one, would you go to Hue or Sapa and why?

Hue and Sapa are totally different. Hue is great for arguably the best food in Vietnam and some nice hotels - La Residence is probably the best. However,the weather is often grey and wet due to its situation between the sea and the Annamite cordillera. The imperial city is worth seeing as are the tombs.

Sapa does not have any great hotels but the scenery is beautiful and the hiking superb. Of the two, I would choose Sapa as a nice contrast to the bustle of Hanoi

Another thought is the Ba Be lakes area. More remote but equally beautiful.

Our blog has some photos and more info on these places:

http://www.travelpod.com/travel-blog/candcthai/1/tpod.html

Entries 13 - 16 cover Vietnam

Some photos of the Mai Chau valley and Sapa:

http://www.travelpod.com/travel-blog/candcthai/3/tpod.html

Hue had the feel of Bejing in the seventies but of a much smaller scale. The tombs and the citadel are the tourist spots in Hue, the city itself is not much to see. If you like to walk in nature, Sapa will offer you that. I saw some pretty decent airfare from Macao to Vietnam( i think hanoi) a year ago. I enjoy halong bay, but it's not everyone's cup of tea.