You will find different answers depending on where you look. Implant companies usually quote up to 25% (you can check the mentor or allergan websites). Some doctors quote under 5 % , I think 9-11% is more accurate. Some patients will be a little firm.. At the extreme , just a few are hard, and painful.
Both silicone and saline implants have the same shell so the risk should be equivalent. The shell is what confers the risk of capsular contracture. That being said, even in 2011 we don't exactly know what causes some patients to develop capsules. One of the few things we do know is that hematomas increase the risk for capsules so please alert your surgeon if one breast is significantly more painful or swollen after surgery.