from what I have read/seen/heard, with 98 ron fuel you will get a more pure fuel and more efficient burn, thus getting more power and better fuel economy (though this may only be minimal depending on your car) also you will increase the life of your engine, compared to 95 Ron.
I ALWAYS use BP Ultimate unless an absolute emergency.
with everyday/non-sports cars it doesn't really matter as much. Sports cars are made to take advantage of better fuel.