Halong Bay or Sapa?

I found Halong Bay very pleasant, 2 nights 3 days including a trek to the top of Catba Island. i didn't find it too busy though it could have been that time of year. The scenery is breathtaking and because you are on the boat for a lot of it you are free from nagging touts and vendors for a lot of it. Sapa on the other hand though while rewarding was a bit touristy for my liking. I would skip Sapa and visit Ninh Binh instead (Tam Coc, Hoa Lu) much more relaxed though the boat crew can get a bit heavy with the hard sell. If you haven't had any exposure to Hill tribes before however Sapa may be the place. You can spend a night in a homestead in one of the villages or take day trips from Sapa itself.

WeiLong in Vietnam

Agree with #3
Both Sapa and Halong are worth to visit. But in 3d2n, Halong will be a better choice.
If you had more time to spend (5 or 6 days), you might want to think about Sapa.
I took a Halong tour last month and had a good time there. The trip was relaxing, and food was delicious.
The company I used that time is vietnam opentour, their website: vietnamopentour.com.vn

Vietnam Opentour are the new APT Travel

I has been there! That's a really cool place. The natural beauty is remained. Last time I visted there with the help of a Vietnamese friend. That's a really nice place.
The company I used that time is vietnam, their website: http://lacviettour.com