Which is better - 90 octane no ethanol or 93 octane 10% ethanol

Ethanol draws water, has a higher octane, has a lower stoichiometry (that is why E85 users need to run what would be richer compared to gasoline) and does not lubricate as well as gasoline. Here in Oregon, we have up "to 10%", so you may never get a true E10. I have never had one issue with any of my older cars or power equipment, even when sitting for long periods with 87, 90, or 92 octane.

Regardless of composition, gasoline will degrade while sitting open to atmosphere. Its octane actually lowers during evaporation.