I'm all over the map on the gas I use. For a long time, I was a die-hard CT gas customer b/c I'd use flyer coupons to get extra CT money.
That got a bit tiring, so now I tend to just go wherever I'm nearest to when I need gas.
That said, I don't know if it's placebo, but I swear my cars runs just a bit better on Petro-Canada regular, so recently, I've been going to PC more. I think PC works really, really hard to offer quality products, so I think their gas is top-rate.
As to differences between gas, the 'base gas' may be the same, BUT, the brand-specific additives added at the station are different. Again, PC's 'Tactrol' additive is a good one, and probably pretty close to Chevron's 'Techron' for quality...