A friend and I actually did a fairly extensive amount of testing with a couple of different cars awhile back, and we figured out that per octane rating, between different companys, you generally get about the same miles per dollar so say chevron's $.05 more per gallon, you get thatmuch better economy.
Thatsaid, she's kicking my ass. I've been pulling MAYBE mid 300's time for new plugs. Hopefully that'll do it.