Some drivers need a very big difference in performance before they notice it. I've done work where we modified a normal road car so that we could switch fuel just by pushing a button on the dashboard. It responded very well to changes in octane. We made no changes to the engine control system at all. Even when we were running fuels that offered quite substantial differences in performance, many drivers could not tell.
We also modified a very high performance modern car to allow us to switch fuels. We then let a professional driver take the car on a track and see if he could tell the difference between a matrix of fuels (we also had a data logger and GPS on the car to cross correlate his comments with hard data). The only person that knew which fuels were which was me. The technicians knew only the fuel codes. The driver was not told whether the fuel had changed or not and had no timing data. The driver picked it right every time.
My advice to anyone is that if you can't tell the difference and the manual says use regular, then apart from improved (or extra) additives in (some) super unleaded fuels there is no strong argument for spending the extra cash. This is also true for diesel.
As for the differences between brands of fuel.
Regular:
In the UK all fuel suppliers include additives that have similar functionality, but vary in chemistry, dose rate and effectiveness. Typically in my experience, fuels from the major oil companies tend to have more effective additives than those from supermarkets. However supermarket fuels are typically fit for purpose.
Super unleaded:
Some fuels achieve the increased octane rating by the inclusion of ethanol. This can be supermarket or Major Oil Co. fuels, but not necessarily all of them. Ethanol is cheap and has good octane quality, but it contains oxygen. It can also be produced using nominally sustainable methods, so is seen as having some level of environmental benefit.
However Oxygen is not a fuel so the energy content per kilogram for ethanol containing fuels is lower compared to fuels that achieve higher octane by including high octane hydrocarbon components. So some super unleaded fuels will give less fuel economy than others. This is a bit of a crap shoot because without analysing it, nobody knows how fuel at the forecourt was formulated.
It's perhaps worth noting that octane (RON) is just an inspection property of the fuel. It's a headline number and not a definition of how well a fuel performs in an engine and there are numerous ways of formulating fuels that meet a certain octane, but then offer different performance levels in the engine.
