I have 6 days and am debating if I should go to Laos (Vientiane and Luang Prabang) or Vietnam (Hanoi and Hue)

Any advice?

Yes. Laos. Breeze through Vientiane on your way to Luang Prabang, and spend your days there. You won't regret it. Check for previous posts on this board concerning lodging and things to do in LP.

Here's another vote for Laos. Skip Vientiane and spend all six days in LP and vicinity.

IMO, save Vietnam for another trip but make the visit for at least two weeks or more so you can add in other cities besides Hanoi and Hue. Add in Hoi An for at least 3 days. 6 days in Vietnam is too fast.

Travel slow in Laos.

Hoi An was a favorite of mine.

We love Laos!!

Vientiane was a bit disappointing for us. We flew from Hanoi and went in to town for a look-see. Pretty boring place compared to Luang Prabang. We flew from Vientiane to LP US55. Road was deemed unsafe when we went in Dec 2003. Check up on google for latest news. The road is called route 13 and passes through Vang Vieng. We've heard there had been some security problems en route from Vientiane to LP for "rich" foreign tourists and some local skirmishes amongst local tribes. Just wasn't prepared to risk it. LP is a great place to relax. Accom is very cheap. Stay in a local guesthouse for US15 with air cond & private facilities. A good one is Hoxieng at No. 40 Ban Hoxieng Street. Or splurge on Sala Prabang on the Mekong river. You could hire a car/driver and go up to Nong Khiaw for a excellent day trip. Water falls and lovely little places on the river to stop and enjoy the atmosphere. Don't expect great shopping in Laos, except for silver and maybe silk & cottons. Not as good as Chiang Mai or Hoi An. I still prefer Laos to Vietnam. Have fun!