ah, at last! someone finally called it by its actual name!
which fuel you use can probably be determined by what mods you've got done to your car.
eg: My car is using the factory ECU and only has a front mount and exhaust, so its no big deal to me to use all the varients of the 98 ron fuels. however, other cars with programable ECU's and more serious mods will be tuned to run a particular kind or brand of fuel and wont perform as well when owners have to switch fuel brands for whatever reason.
having said that, i've also tried V-Power Racing (102 ron and 5% ethanol) and i definately noticed a fair difference, even in my car, but after about 3 weeks of running on only that fuel, my car started to miss and breakdown. because ethanol burns at a higher temp than petrol, i think the spark plugs dont like the extra heat. i only say this as my mate who was using the same fuel as me at the same time, starting having the same problems. once we drained the petrol tanks (absoulte pain in the ass!) and used the 98 ron stuff again, the cars went back to working fine. maybe it was a bad batch of fuel, but it definately leaves food for thought....