The 98 has a higher octane rating then the 95.
Essentially a higher octane rating means that the gas will burn in a slower and more controlled way. An engine burning it will be more resistant to combustion knock.
But...
Modern cars have knock sensor ignition systems that automatically adjust the ignition timing to how the fuel is burning. So in a modern engine you can safely use 98, 95, even 93 or 89 octane fuel. Your car will adjust.
Of course if you do use high octane gas the ignition system will adjust and your engine will produce slightly more power, perhaps 1 or 2% more. Most people will never notice. Keep in mind that this extra 1 to 2% more power will cost you 5 to 10% more money!
Essentially a higher octane rating means that the gas will burn in a slower and more controlled way. An engine burning it will be more resistant to combustion knock.
But...
Modern cars have knock sensor ignition systems that automatically adjust the ignition timing to how the fuel is burning. So in a modern engine you can safely use 98, 95, even 93 or 89 octane fuel. Your car will adjust.
Of course if you do use high octane gas the ignition system will adjust and your engine will produce slightly more power, perhaps 1 or 2% more. Most people will never notice. Keep in mind that this extra 1 to 2% more power will cost you 5 to 10% more money!