Ive recently been buying and comparing different quality milks and comparing the taste and results before and after steaming.
There is definitely a difference. Obviously each region will have their own brands that work well or not, but certainly paying a bit extra for quality milk is worth it.
While the diet of the cows does play a role, I think the other thing to look for is milk brands that use Milk permeate. From what I understand, milk permeate is a by product that is added back in to cheaper brands to make it go further (basically theyre watering the milk down. Not harmful or bad in itself, but it does lower the quality of the milk.
How do you tell which brands add milk permeate? I dont know. It doesnt say on the carton.
edit: I should say, today tonight did an article on this a while back and pura had milk permeate, while the more expensive farm house brand found in Melbourne did not.