It is somewhat difficult to answer this question since everyone has different taste in women. Those who love Laos and hate everything Thai will probably favour Lao girls over Thai girls, and vice versa.
We hear of so many westerners with Thai wives, that's because Thai girls are sweet talkers and they tend to be more westernised than the majority of Lao girls. Also it's easy for Thai girls to jump on you, whereas Lao girls are more conservative by nature and they like to be picky when it comes to choosing a potential husband.
A large number of Lao girls living in foreign countries are married to western men because they are adapted to western society and they interact with westerners in their everyday lives. We are now seeing alot of mixed Lao-westerners amongst the new generation of kids.
These days, Thai guys are coming in droves to Laos to search for a Lao wife. The majority of Lao guys living in Laos definately favour Lao girls, to hear a Lao dude and Thai girl living together in Laos as husband and wife is almost unheard of.
Lao girls are better wife material than Thai girls in our opinion, but not everyone will agree. Thai girls are just too sophisticated and not to our liking compared to Lao girls who are more upfront, they are direct and they are honest in a relationship. In general Thai girls are in play mode who like to go out, whereas the mainstream Lao girls are the type of people who like to stay at home and do household chores, work hard and look after her family.
There's also lots of ladyboys (katoeys) in Thailand, and just looking at a Thai girl often makes you wonder whether she is actually a girl or a guy. Also, the full range of cheap cosmetic and plastic surgery available in Thailand means any Thai girls can change the way they look to unnatural.
On the positive side, Thai girls are sweet and romantic and they are open to meeting new people due to their country being more multicultural than Laos. They are clever at using their charm to make guys attracted to them.
Which girl is better to have as a wife is obviously everyone's personal choice. There's also men who couldn't care less if she is Lao or Thai, as long she meets all the criteria that a good wife should have then they are happy.