Hope this helps.

First, your owners manual explains the octane (old term) or AKI (newer term) requirements:
1) the engine is designed to run optimally (i.e., best) for 91. With this, your engine will run with the highest power output as well as the highest fuel mileage.
2) it is also designed to run on 87, but -- and here's a big "but" -- it does so by "detuning the car", which means that the ignition timing is retarded in response to engine ping (there's a sensor for that) before you can hear it. This ignition retardation reduces power output and fuel mileage.

If you live in flatland areas (no hills) and drive modestly in suburban traffic, you probably won't notice a change because this doesn't stress the engine and cause ping, and the ignition isn't retarded. But if you drive on the highway and/or live in hilly areas where you have to drive uphill, this will make your engine ping on 87, and you'll suffer the aforementioned reductions.

Another responder wrote that higher octane (or AKI) fuel has more beneficial additives. Should you decide to use lower octane fuel, you should know that his statement is only true for some brands of gasoline. There is an organization called Top Tier Gasoline, which monitors the additives of different brands of gas. Those that are designated Top Tier (Exxon-Mobil, Shell, etc.) put as much beneficial additives in the lower octane fuels as in the highest octane fuel. Go to toptier.org and find out which brands do.

Second, regarding ethanol, I'm a boater and boating organizations such as "BoatUS" have studied this thoroughly/scientifically (which you can look up). Ethanol is NO friend of your car, regardless of your engine.
1) It is powerful oxidizer, which deteriorates your fuel system's hoses and seals, and dissolves gum and varnish only to redeposit them where the alcohol volatilizes, in your engine's intake passages, valves, etc.
2) It is a powerful water attractant, vigorously pulling water vapor out of the atmosphere and precipitating it in your fuel tank. It, if it accumulates enough, will lead to phase separation where you'll have actual "globs" (for lack of a better term) or even a whole layer of water laying at the bottom of your tank, ready to be sucked into your fuel intake to the engine.

There is a very good reason why E15 is not approved for car's model years earlier than 2001 (I think; check the warning labels on the pump), small engines (lawn mowers, chain saws, etc.), and most importantly, marine engines of any year! Federal tests have shown significant damage to these. So why would you want to put fuel that is 2/3 of this ethanol concentration, E10, in your car if you had a choice? Most of us don't have a choice -- we can't find an ethanol-free gas station. But you apparently have a source. Doesn't it make sense to exploit that opportunity?