I've only been to Auto Atlantic and SMG. Used AA until SMG opened.
The only problem I had at AA is the long waiting period for a booking, avg a month, except for when its a F1 service that you need, then its a week.
SMG, is fantastic. You could book your car in for tomorrow or in 2 days time.
Regarding a courtesy car. AA, you are lucky if you get 1. SMG, will give you 1 if any aren't booked out.
Both of them are very friendly and good service. SMG is more convenient for me as its walking distance. If I was in the middle of the 2 then I would go to AA cos they are more "BMW" experienced than SMG.
SMG have mechanics from Mercedes etc. They still learning.
For example, I had a question which I asked both, and SMG couldnt answer me properly as AA did.
BTW, the hot chick's name is Nicky, that sits at the front desk of SMG. Theres 2 other babes there at SMG.