The only benefit you will see from 98 is if the car is tuned to take advantage of it.
A stock car will run just as well on 91 as 98, the only time higher octane is required is A. if the vehicle has been tuned for it or B. the manufacturer specifies it which you can usually find behind the fuel filler door (usually due to a higher compression ratio on modern small engines).
If your car is tuned for 98 though then usually it's due to the fact the tuner has bumped up the timing in the maps to take advantage of better knock/denonation resistence, in that case you do really want to run it.