I recently got a Crux and I agree with northface's post that the Sinister is slightly longer and that it acquires "lane shine" quickly. The biggest difference for me is the backend reaction; even though I had both set to 2000 surface with similar layouts, I felt the Crux gripped the lane more and charged to the pocket. I leave a fair amount of ringing 10's with the Sinister, but my carry with the Crux has been phenomenal...I have never seen so many messengers and the 6-snapping-out-the-10 in my life.
However, both play an important role in my bag and are keepers. The Crux is my new first ball out of the bag and the Sinister I can use for longer periods of time while maintaining that strong backend on heavier patterns.