Upvote:1
Assuming you're going to Laos, the answer is neither: you'll get the best rates buying Lao kip in Laos itself. The currency is minor, not really convertible, and unstable, so exchange rates outside Laos and maybe border towns will be terrible and many changers don't even offer it. Even in Laos, larger purchases including visa fees are often quoted and charged in USD, THB or CNY, depending on where in the country you are.
If for whatever reason you absolutely have to buy kip before arrival, one of the major dedicated exchange chains (Superrich, Vasu, etc) will be your best bet and certainly better than the airport.