Firstly, Australia and Europe use a different definition for Octane than North America, so be careful of some of the responses you may get on a US centric site like this. Your 91 is not our 91.
Next, go with what it says in your owners manual or on the sticker inside the fuel filler area. Internet experts do not know more than the engineers who designed your car.
Higher Octane is not always better. Most people misunderstand what an Octane rating is. Higher Octane is NOT cleaner burning or more powerful. Higher octane means that it is less susceptible to pre-ignition. In a car that compresses the fuel-air before injecting it into the cylinder (turbo or supercharged) this is very important - it stops the fuel from combusting outside of your engine block. In old cars with worn engines, high octane is useful because it prevents detonating the fuel in the cylinder but before the optimal time (when a cylinder detonantes before the right time it pushes the cylinder the wrong way and produces "knock", which you can feel and hear in the car).
Generally speaking the more 'tuned' an engine, the higher the need for a high octane petrol.
Low Octane has an advantage, it ignites easier, and as long as it isn't igniting too early, this is good for you. It means it will burn more completely, so you'll get more out of the gas than a higher octane. That is why car companies always recommend using the LOWEST octane the car is designed for.
Higher octane petrols are thought of as cleaner because they tend to have more cleaning agents included in them. There are two reasons for this: one, since they may not always burn as clean, more cleaning agents are wanted; two it allows the petrol companies to justify the higher price and sell to more people.
Each petrol company puts in its own additives. And each engine is designed for certain burn characteristics. This means your car may in fact run better on Company X's 95 than company Y's 91 octane. But it may run best of all on Company Z's 91 octane. My old Mazda 6 did not run smoothly on ethanol. It ran best on pure petrol (10% ethanol being the standard here now). My car before that had a very old and dirty engine and ran best on a high octane high ethanol blend.
Ethanol has less energy in it than petrol. So a 10% blend will have less energy than a pure petrol. But ethanol has other advantages, it lowers the freezing point of the fuel, and it is a very stronger cleaner (will help clean your fuel lines and injectors). If your goal is purely fuel economy, on a new engine you'll probably see a few percentage points better fuel economy with pure petrol (I got 5%-10% with my Mazda, but it was an outlier). And if you have an old engine, you may find that the benefits of ethanol outweight the loss of energy content (I got 2%-5% better with ethanol on my Acclaim that had 220,00kms on the clock).
Track you mileage (I recommend a tool like Fuelly.com) and try different petrol stations. Eventually you'll find what works best for your car.
