91 (highest commonly available at upper elevations). This is a "favorite" topic on the forum. Some on here have claimed higher MPG with regular gasoline than they saw with premium gasoline. I'm not believing, but hey, everyone has their own world view, not always rational.
With any turbo or high-compression engine, you generally want to use as high as possible to prevent knock. Most engines will pull back to prevent it, but this is generally not 100% either. The more extreme your environment as far as heat and humidity, the more this is an issue. The secondary concern is the loss of power and efficiency. There are a few engines of this sort that specify regular is fine and higher octane is not preferred, but they are usually pretty specific to this. It's not a giant conspiracy between car makers and the petroleum industry as a few people tend to think.
The way it was explained to me years ago when I modded talons was that higher octane fuels burn cooler, which means slightly less energy per explosion. BUT... as soon as the engine starts seeing a reason (knock) to pull timing and add fuel, those effects greatly reduce performance.
So the answer is that it depends. You want to run the lowest octane you can where you don't give the computer a reason to step in and dial things back.
So those with a stock setup should be running less octane than those who have modifications. The people with mods must run higher octane to avoid giving the computer reason to step in and try to protect the engine.
Typically for stock cars the octane recommended by the manufacturer will be sufficient to avoid any dial back and so should out perform higher octane for both power and mileage. If you can log to make sure that's ideal. No dial back, no reason to pay more.