Just rate of burn and I used to always run Premium too thinking it was better for my engine, it isn't. From what I have learned you just run the lowest octane you can without the engine pinging. The lowest one that doesn't ping should be giving you the best performance for your engine. Use high octane in high compression engines.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!