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.
High test is designed for higher compression engines as a defense against pre-ignition. It is slower to detonate. ( This is why we used to burn it in the older "carborated" engines when they suffered from Knock ) Net gain in a Terrain, none. Use the mfg. recommended.
I've experimented around between Ethanol(E10) and pure gas, 87, 89 and 91 octane...And not just in my V6 Terrain..
I've come to a conclusion that E10 is crap for mileage and power gains..It will work, as modern engines with their ECM's will compensate a little for the lower BTU content of alcohol..But there is a limit to what they can do.
I found for winter months, where it is 40 to 0 degrees F, or lower ambient outside..that 87 octane is perfect....In summer months/temps the 89 octane is perfect for our Terrains..any Higher octane will render no performance gains, so is a waste of money.
I use top tier (Conoco, Shell, etc) Pure gasoline.