I would chose Ghostly Whisper over Gravity Crush.
The reasons are quite simple. Ghostly Whisper has a longer duration [18s] than it's cooldown [10s] while Gravity Crush has a 3s cast time and 15s cooldown.
The con of these two skills are they are both stationary.
This makes Ghostly Whisper a better grinding skill in dense spots compared to Gravity Crush.