Using brim-to-brim re-fueling is the only way to get accurate MPG. And I think you need to average that over several fill ups.
Compared to brim-to-brim I've found the on-board MPG average display a little optimistic, but should be consistently so for comparison. Having said that, driving conditions even over the same journey can vary somewhat day-to-day in my opinion (traffic, weather, detailed driving style, etc).
All this makes it very hard to do anything other than anecdotal comparison of fuel sources, given than at best the likely difference between them is only going to be a few % in MPG. Trying to drive consistently shows my tank-to-tank variation in that order anyway using the same fuel... :-(