Was trying to decide between Vientiane and Luang Prabang and ended up with the latter because it's supposed to be so pretty and charming and is so highly-ranked in travel lists I saw.
But I do suspect that, depending on what I was in the mood for, I might have at least as good a time in Vientiane, since it's bigger and more bustling and lively.
In any event, I've heard Laos is AMAZING. I'm tentatively planning an extended SE Asia thing next year, and if I do, I'm definitely headed here.