Supermarket fuel is garbage, I tend to avoid at all costs unless its the works van.
The main benefit of premium fuels seems to be the added detergents they put in to keep the engine and fuel systems cleaner, though you are likely to see a benefit of this only through prolonged use, not just a single fill up of premium once or twice a year.
That being the case, I would think premium fuel would be of more benefit to diesel owners as it tends to be a dirtier fuel in the engine.
If you are after economy benefits, it's a bit of a grey area.
I'm not sure about diesel as I've never owned a diseasel, but with petrol, a higher RON fuel typical of premium unleaded allows you to more comfortably change up a gear at slightly lower revs due to an increase in low down torque.
I am adamant I can feel this in my 75 - running on supermarket fuel it just feels lazy so you have to give it an extra 200-300rpm before shifting up, particularly noticeable when on a slight incline.
This is where you can in theory save a few pennies on your cost of fuel as changing up earlier means you keep the revs down which in turn means you are using less fuel. Whether it outweighs the additional cost of putting the premium fuel in your tank is another argument altogether.
I don't believe there is any economy benefit to using a premium fuel if you are sitting at 70 on the motorway all the time - the benefit is to be had lower down in the rev range.
Personally, I only use premium because I like the idea that it keeps the fuel system and engine cleaner than a cheaper fuel, which I appreciate when running an older set of vehicles such as ours.
I have read many stories about people that struggled to get their older cars through the MOT due to failing the emissions tests, but a good use of premium fuel beforehand seems to get it through.
I previous had an Alfa Romeo MiTo with the TwinAir engine (900cc, 2-cylinder, turbocharged) and it ran like absolute shizz on supermarket fuel and sounded like a bag of spanners - it was notably better on premium.
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