Firstly, the Athenians came really close to winning in Sicily and had incredibly stupd commanders. And Philip will have better cavalry and better commanders than the Athenians did anyway. The hard part of Sicilian conquest is to keep from getting entangled with Carthage, I'd think. As DG says, Sicily might make a good conquest for logisitical reasons.
Also, didn't Alexander accuse the Persians of plotting his father's assassination and use that as part of the reason for his assualt? I seem to remember that while Philip's army is in Asia Minor, a full on war with Persia is yet to come. Without the assassination, Philip may bide his time to find a more convienient casus belli, as he did in his wars in Greece.
I agree Nicomacheus... If Sicily gets in line with Carthage then Alexander has an enemy on his rear and without Sicilian grain he would ran out of supplies soon... He has to rush to Egypt fast so he can set up a new supply base there... I wonder how will Carthage react if after Sicily (which would have given them enough supplies and a naval base) to the conquest of Egypt...
Would they felt strong enough to attack Alexander there?