Wit's End gives 40% AS and +42 Magic Damage
Ionic Spark gives 50% AS and every fourth attack procs a chain lightning
Several builds for AS focused champions such as Irelia recommend a Wit's End, in her case it also benefits
Bladesurge which helps with killing fleeing enemies. However, in the general case and if I am trying to adapt my equipment to the situation (and I have an AS focused champ) - how to decide between them?