I think that Michael killed Sollozzo first because Sollozzo was already nervous. He had obviously expressed some discomfort over Michael's trip to the restroom, while McCluskey waved it off, saying Michael had been frisked thoroughly. As Plaw pointed out, McCluskey believed that his position granted him immunity, while Sollozzo was a man who lived dangerously, and therefore suspected danger everywhere. Also, although it was McCluskey's punch to the jaw that released Michael's rage, Michael knew that his business was with Sollozzo.