I like both of the brands you mentioned but I do more Chinese and Vietnamese cooking than Thai cooking so I like Flying Lion the best. It has a bit of a different flavor that's more reminiscent of those foods.What brands of fish sauce have you tried? Three crabs is good (although it has hydrolyzed soy protein aka MSG) and Red Boat is probably the best (no MSG).
Theres no MSG added but I think it does have some naturally occurring MSG, but I'm pro MSG so I'm cool with it lol. I have a bottle of Ajinomoto Ajipanda in my cabinet which is essentially an MSG made from fermenting raw sugarcane molasses.