Well, you only have two choices, BMW of San Antonio or BMW of Austin
Personally, I have been to BMW of San Antonio multiple times for service work, and have always been treated fairly and professionally. I always schedule service appointments with Mike Brehm, he is top notch.
Never been to BMW Austin.
There are plenty of independent repair shops around the area too, the only one I have had experience with is DBC Tuning in Northwest San Antonio, and they do great work.