Factory vs aftermarket extended warranty

I am interested in understanding this also. I just purchase a 2012 CLS63 AMG and the dealer made a hard push to sell me the factory extended warranty but a friend suggested I wait until nearer the end of the new-car warranty period and see if the car seemed to have problems and that I could buy the aftermarket warranty at that time. He didn't think the pricing would be much different but wasn't sure. Hard to make decisions without facts...........