That is true, but it has little to do with advertising. In the absence of other evidence I buy the product that costs least per gram/litre. In most cases that's the supermarket's own brand, which is also the brand I've most likely to have heard of, because it's difficult not to have heard of Tesco or Sainsbury's when you're in Tesco or Sainsbury's. The choice between Tesco and Sainsbury's is determined by location and transport options. They're not usually right next to each other so that you've got to decide between them on their merits.

Note that in the case of consumables, like shampoo or toothpaste, there's almost no risk in getting the cheapest: if it turns out to be crap you'll mentally blacklist it and buy the second cheapest next time if the blacklisted product is still the cheapest. Buying a car is a very different matter.