The Rana is a better way to go, as it does have the extra core. The advantage of the 550 is that it can be unlocked. So then you will basically have a 955, in a sense. So if your willing to take the chance the 550 is the way to go. But if your not going to try unlocking then go with the Rana, as its not 100% it'll unlock.
Here are some reviews Tom's Hardware did with the Calisto and Ranahttp://www.tomshardware.com/reviews/athlon-ii-x3,2452.html