Attila wins, assuming we are talking about a set-piece battle. Roman armies during the period in question, if you will recall, had a very hard time dealing with the Parthians, who, like the Huns, fielded a cavalry army primarily composed of horse archers, which the Roman legions were NOT equipped to deal with at that period of history. The Huns will be an even more formidable opponent than the Parthians, because they had an improved composite bow and had incorporated large number of Germanic (Ostrogoths and others) armored lancers into their armies as well, so they had improved firepower and also an excellent shock arm. Basically the Huns will treat Caesar the way the Parthians treated Crassus at Carrhae, only worse, and there's very little Caesar could do, tactically, to change the outcome, given the limitations of his troops.