EU Directive states minimum of 5% Ethanol in all gasoline pumps on the forecourt. I believe Tesco used to have a higher content of Ethanol than this in their fuel.
Ethanol can suffer phase seperation from fuel (esentially when subject to enough water it globulates and sinks to the bottom of the tank) if ethanol is used as an octane booster and phase seperation occurs, you're left without the additional octane.
So, you can say, the higher the ethanol content, the more unstable the fuel is regarding storage.
Real life says, best fuel is freshest, so whichever has a busy forecourt selling quantities of premium unleaded will probably be the better bet.
My car usually knocks with Tesco momentum, it doesn't with Optimax/V-Power/Vin Dizzle Nitro.