My suggestion is to start with the one closest to where you live/work (or most convenient) and make friends with your SA/tech. If they don't treat you well, then move on to the next one.
Everyone will have both good/bad things to say about any particular dealer and there is no dealer in this area that truly walks on water. It's all about the relationship with your SA/tech; you kiss their ass, and they will do the same. If there ever comes a time when you need them to stand up for you against BMWNA, the first thing the GM or Service Manager will do is ask for SA input and history.