Well ethanol is actually not bad for the engine, just for the rubber in the fuel system. Its a much higher octane(dont let the number at the pump fool you), therefore allowing a lot more timing and ability to create power, but requires a much greater amount of fuel(runs at like 10:1 for stoich instead of the 14.7:1 that gasoline does.) Downsides are it eats rubber so you need to buy special stuff and it sours a lot quicker than standard gasoline. That is why non ethanol gas is good for boats and lawn equipment and the like since they do a whole lot of sitting around. As far as octane goes, the lower the octane the faster and more potent it is, and the higher the octane the lower the burn rate is and is better at combating detationation. This is why high compression motors and boosted motors require much higher octane, they are more potent and would burn so fast it wouldnt make the proper amount of power. There are a lot of different factors into the type of fuel and octane. Our trucks on stock ignition and timing settings are not designed to run higher octane, but with higher compression, boost, or increased timing, you can take advantage of a higher octane and get more power out of the same amount of gasoline, therefore making it more efficient. The cost difference is also MUCH less than what you would think it is for a 15 gallon fillup, a difference of $0.20 ($2.59 for regular and $2.79 for premium) per gallon would net you a cost difference of $3.00 to fill with 92 octane instead of 87 octane and greatly reduces the chance for pre-detenation and engine knock. Granted if an engine is only designed to run on 87 octane you will not see any performance gain(usually a loss due to incomplete burn since it doesnt burn fast enough) This is only the basics and the start of the differences and different advantages