If it really is ethanol-free, the 90-octane is definitely a much better deal, since you should get both marginally better mileage and it's cheaper.

Many people in various states (like mine) can only get 91 octane gas at many stations and it doesn't seem to knock or cause the ECU to back off, so I think you'll be fine on 90.

On the other hand, the ethanol is almost certainly not hurting anything besides a marginal hit to your mileage as long as it's quality - cars built in the last 20 years were designed with E10 gas in mind and have E10-resistant hoses/pumps/injectors/seals. The risk with E10 gas (and why it still gets a bad rep) is that it becomes unstable and picks up water over time, so a station that doesn't get filled frequently or has poorly sealed tanks will deliver bad gas that can cause damage.

The chances of breaking a modern engine with an intelligent ECU via gas that's one octane number lower than usual are slim to negative, especially an N/A car. Knock detection and compensation has gotten quite good. If you notice a loss of power, just don't use that gas again!