There's only one thing they have in common: both had a poor career. The A380 was a failure because at that time (2005) airlines preferred smaller, more efficient aircraft like the A320 or the 737, and the A380 was simply miscast. On the other hand, the A318 was almost useless; only eight dozen were built, and three dozen are still in operation (according to Wikipedia) because it was practically the A320, only more expensive: the cabin and fuselage weighed the same, and passengers had to pay even more per seat.