I have no facts to back this up, but from my anecdotal evidence, I've found that quality is the most important too.
Before I bought my FEH, I had a '98 Mustang GT (big switch, I know). It was a stick, and every time I put "cheap gas" in it, it would not idle without me pressing on the accelerator. I'd push in the clutch in or put it in neutral, and it would die.
I noticed that it only happened a couple of days after I went to a Speedway Gas Station (I don't think those are everywhere, but they are cheap). And it would start acting normal after I ran it for a while. So, after I figured it out, I never went back, and would never consider putting it in my FEH.
It could have been a ethanol content thing instead of quality, but I'll never go back. I only go to Marathon, Shell, or BP now.