I use premium and have been since my first fill up with my car. Premium where I get gas is ALWAYS exactly 10cents over regular grade. and mid grade is 1 cent more than regular. I don't know what a difference regular would make in my car since I haven't put any in so can't comment on any differences.
2012 Oxford White Focus Hatchback
Dyno #'s 164.35 HP 147.12 TQ