I would recommend HBL over Rockville Porsche. Their prices are usually better, since Rockville is a very small dealer and hardly budged on their price when I was looking to buy.

In addition, the service department at HBL is second to none, they will give me an appointment usually for the next day, while Rockville Porsche would give you one for a week later.

Another advantage to servicing your car at HBL is that they will give you a FREE loaner, even if you didn't buy the car from them.

I have been using HBL for service on my Mercedes and Porsche, and have never had any problems with them.