I'm everyone's been asked this question at least once. But seeing as how most engines made in the past decade are engineered to take both even though a lot of them recommend using only premium. So whats your 2 cents about about this topic?
Just go with whatever the manufacturer recommends. They always have actual data to back up their recommendation. Its not like they have a stake in the gas companies. If that was the case then every manufacturer would only recommend premium.
I would just use premium. Not because people say you get better mileage. Not because people say it makes it run faster. But mainly because most premium gases don't have ethanol mixed in. Also premium gases can remain stable in your tank way longer than regular gas. Meaning if ever for some reason you're unable to start it up for a long time it wont gunk up the fuel lines.
I use premium because it gives you more miles per gallon than regular gas. It has special or extra detergents in it that are worth the added cost. Using regular gas in a car designed for premium will definitely damage the engine.
Octane is defined as a fuel's resistance to knocking. There is no benefit if the octane is higher than what the engine needs. Engine knock occurs when fuel in a combustion chamber ignites before it should. This disrupts the engine's operation. But electronic knock sensors are now common and have nearly eliminated engine disruption.
Premium except in the hottest of summer here in 'Vegas when air density is low, as in higher altitudes. Then I use mid grade gas.
Premium gas takes advantage of the higher 11:1 compression ration of my 1.8 L. engine. It gives me more power! (according to my dynamometer)
But regular gas dyno readings were taken when the car was delivered and the engine was not yet broken in. I'll do more testing at 8,000 miles.