You are likely to get the good old "use the search function" really quickly, but from all I had gathered when I did all of my research, there seems to be a consensus that either works very well.
There are a few die hard Calvert guys that will tell you they hold the record, but the fact is Assassins are not on nearly as many vehicles. Bottom line, the Assassin has a lot more tuning available, but from what I have heard, once they are set, you are pretty much good to go like a Calvert.
I ended up going Assassin, and while I have not got to the track yet, (hoping to this weekend) I can tell you that I am more than satisfied so far. Even on the street, the defference between what I had and what I now have is night and day. I never suffered from traction loss, but a very poor ride and extreme rear separation on launch. Now when I get on the car, you can tell the rear is staying put and the car has much better grab even on the street.
Bottom line, take your pick. Either way you should be happy.