I've fooled around with a load of different fish sauce brands in my time, more or less picking the bottle that was at front of the shelf in the store.
But after getting serious on Thai cooking, I've gotten more conscious about fish sauce as well.
Maybe it's just me, but I find the Vietnamese brands milder, more fragrant and varied in taste.
Some Thai varieties are tounge-numbingly salty and shallow in taste bordering on rancid.
Maybe it's just that most Thai brands available in the store here are run-of-the-mill stuff from large producers, whereas Viet brands bear every sign of coming from small family producers and are usually hidden at the back of the shelf (in a unmarked brown paper box).
On the other hand I've heard that some of the best Thai brands are not available outside of Thailand.
Anyone with more experience on this than me?