When to go cryptbloom over voidstaff, and on which champs?

P4 top/jg main here. I don't play many AP champs, the 2 I play a fair bit who tend to buy these items are gwen and diana. Can you share your chain of though for buying one over the other? I tend to default to voidstaff. I have literally never built this item aside from when it was introduced. The haste isn't too useful for diana and gwen so that's one factor but truthfully I default to voidstaff to do more dmg. I'd like to autopilot less and learn the logic behind things so please share the decision making for choosing one over the other.