Octane rating is simply a fuel's resistance to pre-ignition (knock). It doesn't really matter how they get there, ethanol or no, so 93 is 93 and 91 is 91, regardless of ethanol content. For our cars, always go with the highest octane rating, so I would use the 93.
Now, you will notice a hit to fuel mileage with ethanol, and the higher the percentage of ethanol, the worse the mileage gets.
You'll just have to decide what's best for you, but knowing how much these cars knock on the factory tune, I'd go with the 93.