Anyone using high octane fuel in a car which has not had the engine management set up to gain the benefit is mad.......its generally 10p per litre more expensive than 95 RON (at least it is here in NI).
I run my Caterham on 98 RON, usually BP Ultimate as we no longer have Shell stations in NI, but the Caterham's engine management unit has been mapped specifically on a rolling road for the higher octane fuel. The higher octane fuel allows you to tune the engine closer to its limits without risking detonation. A standard road car is nowhere near that level of tune. Some high performance cars will advise to run the cars on 98 RON, usually the high powered Subarus and things like that which again are highly tuned.
I've also been told 98 RON fuel looses its effect if stored for too long so unless your local filling station is selling enough of the stuff and it is kept fresh then again you may as well not bother.
With regard to standard fuel I suspect they are all much the same, although I have been told different retailers add different additives, certainly I have heard many stories of people having problems with supermarket fuels.
Personally I fill up wherever I'm getting low or wherever is closest, sometimes thats with a fuel company eg BP, sometimes Tesco and I've never in 17 years of motoring had any problems with fuel.
The 500 will not benefit from using 98 RON so save your money and spend it on something more interesting
I'm not an expert on this, just relaying my own experience both in my daily every day cars and high performance cars.