Always use premium, whatever is the highest rating (at least 91, 93+ AKI is preferable and advised by Subaru to be used in STis). As for ethanol or non ethanol, it's your choice. Ethanol is known to eat away at certain rubber components, but modern cars are fine to run E10 fuel. I think it's pretty much standard in the US now, BTW.
There are known fuel mileage decreases when using E10 over "pure" gasoline, so it's up to you on what you'd like to run. Just maintain the recommended octane rating and you'll be right as rain .