It's a tough nut to crack-

Variations between most brands within a small geographic region are most likey from the pump shutting off at a different point each time.

Some brands (the 'toptier' group) use 8-10% ethanol for all grades. ETOH reduces FE by ~3% for every 10% mixed into gasoline. See this for detail and what I found out about toptier:
http://www.greenhybrid.com/discuss/t...hlight=toptier

Real variations among brands may exist as some companies pre-emptively add ETOH instead of MTBE, start to distribute low-sulphur fuel in the near future, and generally start to respond at their own paces to new fuel requirements from the 2005 energy act.

If you see a tanker filling the station, go to that tanker company's website (Carroll, Eastern, TGO, etc) and see what brands they carry for clients. They all also carry generic fuel, sold as noname, Osprey, Spirit, etc (around Baltimore).

Not all branded stations are required to carry that brand of fuel- depending on the owner/franchise agreement.

But, if you decide that the fuel at one station seems better for whatever reason, go ahead and stick with it and don't look the gift horse in the mouth!