No one will notice the different as the effect is over time. Dirty injectors and inlet valves with low additives fuels (supermarket).
It's worth paying 1p a litre more for decent oil company branded fuel.
If it was that simple I would.
Nearest Shell is 10 miles away and premium priced as a result of its location beside the A38. The smattering of BP garages we have are 7-10p a litre more unless you catch them on one of the bizarre 'Cut Price Fuel' days when they match the supermarkets.