Using either 97 or 99 octane fuel will be fine.
The basics of it are the higher the octane fuel, the less fuel is required for a given amount of oxygen, or equally more oxygen can be burnt in a cycle with a given amount of fuel. A key element of higher octane fuel is it is more efficient in the reaction with oxygen, producing less heat in the process. This has the effect of reducing engine "knock", a term given to un-burnt fuel in chambers igniting out of cycle, causing the engine to fire out of sequence and thus reducing efficiency. In short, if you can get 99 RON fuel, I would take the hit and travel, I certainly do myself. I'd rather the extra time and miles of getting 99 RON over using lower RON fuel.