“Petrol” has the unique-word advantage: if it’s usage as short for petroleum went away, then that’s it. Whereas “gas” has various uses, including one that is rather confusing given that gas-short-for-gasoline is a liquid. Also, I tend not to like one syllable short-a words, for if a word is only going to have one vowel-sound it’s the harshest one to have. But gasoline is also a simple and very common thing, so having a very simple word as its everyday form makes sense, especially since it ends up getting combined into a lot of short phrases (“step on the gas”, “gas station”, “gas guzzler”).
Jeremy’s winner: gas