It’s one of life’s great conundrums (at least of you are a multimillionaire petrol-head): the iron fist in a velvet glove of the Aston Martin DB9, or the iron fist in an iron glove of the Mercedes SLS? On behalf of Evo magazine, Tiff Needell takes both cars to a wet track and wrings their necks as only a racing driver can (unlike certain TV presenters who have been known to use a driving double).