Sollozzo was far smarter than McCluskey and had sharper instinct. Sollozzo did not like the idea of Michael going to the bathroom in the first place....McCluskey was too busy eating his veal, and just said "If you gotta go you gotta go." Solozzo even frisked Mike even though McCluskey had already done so....which led to the great line "I've frisked a thousand young punks." I think he his Sollozzo first because if he'd his McCluskey first Sollozzo would have had the reflexes and the time to hit Michael...who knows, Sollozzo may have been packing heat himself.