Have been to Chiang Mai twice (once to trek) and Laos twice.

Laos is the geniune article for much of what USED to be a drawcard for Chiang Mai. Chiang Mai is still OK for seeing Doi Suthep and can be good value for creature comfort, but the hilltribe thing has been pushed father and farther out and is very "domesticated" and used to seeing tourists. In Chiang Mai the challenge is to find a trek that gets you far enough out in the boonies that the people you see haven't gotten totally changed by the tourism trade. Thailand has built roads ever deeper into mountain areas and has pretty much wiped out the opium trade within its borders.

Laos definitely feels like it's less tamed and with more of the real thing much closer by, enough so that wandering into areas the Hmong don't want outsiders to go to is a danger (you need a good guide).

Definitely see Luang Prabang before it's ruined. My trip was in 1998 and I'm so glad I got there before Bangkok Airways did. They charge a premium to bring in people with more money than time, and those people have the worst footprint as far as demanding unrealistic luxury or completely messing up locals' impressions of outsiders. LP is very picturesque setting and every other person is a Buddhist monk. The boat tours on the Mekong used to be quite good, but today you likely have to go farther afield. Though nothing like Chiang Mai.

For a really unique Lao experience, take Lao Aviation someplace with little or no road access. Xieng Khoung and the Plain of Jars was memorable some 8 or so years ago. You had to pre-order meals at the guesthouse the same day because there was no refrigeration, and power only went from 6-11 p.m.