I choose www.1sourceautowarranty.com.
I bought the policy when my 2000 S430 had under 9,000 miles and 10 months old. They gave a better price if you buy before the car is one year old.
So far they have been great. Two warranty claims. (I have exceeded the milage limit on the factory warranty) 1)passenger side window motor failed. 2) Moon roof motor failed. Service writer told me that they got a quick response from 1source and the claim was handled promptly. There was a $2.50/hour difference between the dealer labor rate and what they would pay. It was not worth quibling over.
I think they are comparable to starmark warranty and being a second owner buying from a private individual I wasn't eligible for a starmark warranty.