I've nearly always use Shell as there are 4 with 2 miles of my house, so whichever direction I go I can fill up easily. Yeah it costs a bit more but it's convenient and the advert says the engine will be better treated due to the additives
In my old E39 M5, it made 12 more hp on the same rollers with tesco momentum, not scientific as different days etc but that is a big variance for the same car with no changes apart from fuel between the runs, about a month apart on a standard car.