This may be interesting from the scientific point of view - and especially when weight is just as an important factor as volume (which it is, say, for airborne transport). However, for road transport, so far we pay only for volume.
We pay...
We pay the oil companies for their costs in delivering the stuff to us: their prices adjust the whole time and petrol is normally cheaper - by volume. So their prices probably are a reasonable reflection of the technical difference. It happens that mpg is always going to be simpler and more convenient than miles per unit of fluctuating currency but it still doesn't make mpg a good comparison.
But the government puts a tax on it. Most governments deliberately put more tax on petrol than diesel, which inevitably improves diesel value for money.
Miles or km per MJ is probably the most scientific for comparison(or even MJ per mile/km), but with the coming of electric vehicles will probably stick with miles per kWh.
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