which out of the two shell petrols are better overall for mpg, my beaten wallet and general running of the car?

or.... is it just worth using tesco 99 oct petrol?

I'm not sure that your ECU can take advantage of anything better than 95 octane anyway. Many ECUs will be able to retard ignition if low octane fuel leads to pinking (don't try this on JDM Scoobies!), fewer are able to advance the ECU to run higher octane fuel more efficiently. I can't see either having any real effect on economy.

Neither will be as effective as changing your driving style.