I was involved in that fight
All things equal, 91 octane E10 makes the same power as 91 octane non-E10, but the engine has to consume more E10 to make that power, hence the reduction in fuel economy.
The advantage comes with the octane. 93 octane anything makes more power than 91 octane. To my knowledge, you cannot buy 93 octane pump gas without it being E10. If you can, non-E10 would be the better choice for a better balance of power and economy.