> "Personally I find a favourite brand and stick with it."

Two responses:

1) And by what process exactly do you find a favorite? Clairvoyance?

2) If you can have a favorite, then the different brands must have different qualities. And if different brands have different qualities, then they will be differently good for different uses.

> "There isn't a single product I can think of where I'll buy the same thing from multiple brands."

They aren't the same thing. They're differently-prepared multi-ingredient products. They're often not even the same species of fish, because "sardine" is a general category not a species.