Price is an important factor and it's psychological impact should not be underestimated, but there's more to saving money on fuel that price per litre.
What we're truly interested in is the price per mile, and to find this you need to know what economy your vehicle gets when running that fuel. If it's 5p a litre cheaper, but works out 0.5p a mile more expensive (because you get worse economy) then you're actually losing money, not saving it.
You also need to factor in the journey to the fuel station, we used to know someone who drove his Mercedes 15 miles to Asda to save 5p a litre vs the Tesco 5 miles away.
Giving his Merc a generous cost of 25p a mile, he'd have needed to buy 100 litres of fuel just to break even (10 mile round trip to Tesco = £2.50, 30 mile round trip to Asda £7.50, difference £5.00/0.05 = 100 litres). In a car with an 80 litre tank he was burning money whilst trying to save it.