if your car is not pinging on 91 then you could try the 100 it will give it a little more if the engine is leaning out a little in the RPM range. the big benefit would be the ability to add more timing to the engine before detonation(ping) meaning you could turn up your vacuum advance more or add more engine timing.
is it worth it? no not really, i threw some 106 in my tank once it reduced a pinging issue i had and gave me a little more power since the gas wasn't pre-igniting in the combustion chamber, but that was more due to tuning problems i had in the engine itself.
higher octane means higher temperature burn level so if the 91 is on the edge of pre-ignition(detonation,ping) on the compression stroke then 100 would stop that from occuring and give the engine more power because it the pistons would not be fighting against the gas igniting at the wrong time. pre-ignition/detonation/ping, is like a hammer slamming against the top of the pistion as it is trying to come up on the compression stroke just before ignition and the power stroke, so if the pistion doesn't have to fight against this extra force(if it exists) then you gain back lost power.
for the street its better to tune to the cheap available fuels, in my area: i think 100 is in the 5.30$ range a gallon 106 was like over 6 last time they had 108 and 110 as well but those were pure race fuels and like over 8$ a gallon. there is just no way your going to fill a 20 gallon tank on that stuff and tune your car to drive on it all the time.