As the name says, E20 has 20% ethanol and what is not in the name: it is at least 95 octane (knock strength).
91 should have vanished from the market since long but they again stretched it for whatever reason.
As I never use 91 I can only tell you that I need about 0.3l/100km more with E20 compared to 95. About 7.5 l/100km compared to 7.2l/100km.
Currently it hardly pays to use E20 over 95 (only 1.5 Baht difference).
(sorry you have to do US/UK math yourself, I am hardcore metric).
Don't spend a thought on using 91 with a modern motor (high compression ratio).
You just loose power. 91 is for lawn movers and scooters at best. But why go for it with this small price difference?
Even son refused to use it for his brush cutter.
Village "tank stop" has no 91 barrel.
91 octane has been abolished in most of Europe for more than 10 years.