I would say dealerships are more familiar with the common faults with the various models since they deal with a higher volume of cars and they get updates from BMWNA. However, their prices are ridiculously outrageous! And dealers are usually slimy dishonest cut-throats!!!
Independent shops might have to deal with guess work and trial and error to fix some problems. And some of them do not have the equipment to do stuff like diagnostics. If you can find one that's honest and reputable, and has the software to run diagnostics, good for you!
2012 Porsche 911 GTS
2008 ford F150 Foose Edition