I have read that ethanol is not as potent as gasoline, so that the more ethanol you have mixed in the gas the weaker it is. The problem is, I can either get 92 octane pure gas or 93 octane with 10% ethanol. Which, in your opinion, would be better for the Evo? I don't know which is more important, the octane or the "purity."

 

Ethanol doesn't make the gas any weaker as far as octane (or detonation resistance) goes. Ethanol is just one of the various alcohols that can be used to oxygenate fuels. All an oxygenate does is create a better burn "scenario" if you will, allowing the air/fuel mixture to burn more completely being that there is more oxygen in the fuel (ethanol is 35% O2). It is used to reduce emissions in cars that would otherwise perhaps not burn as much of the a/f mixture in one complete cycle.

The octane rating is R+M/2 which means the octane was determined after all the oxygenate blending and such, so you will do better with the 93 than with the 92, even if it were 20% ethanol and they could blend it to 93 octane levels.