I've driven the 325i with iDrive and my 320d has the Garmin/BMW Navigation Professional. Both have bluetooth linked to the car's audio system including voice dialling etc. However there are differences. First thing you notice is that the iDrive screen is obviously bigger, 8.5 inches vs. 4.3 inches is a huge difference. However since the GPS doesn't take up all the screen maybe it's not that big a deal.
From a map perspective, the Garmin version especially if you're using malsingmaps or malfreemaps has far more updated info with a lot of relevant POIs as well compared to the iDrive. It's also updated almost monthly by people actually driving in Malaysia so I much prefer the Garmin version.
However one thing the iDrive GPS has that could be a huge help is dead reckoning. When the Garmin loses satellite signal eg. in a parking garage or tunnel, then it's lost. The iDrive GPS will actually use info from the internal compass and odometer to actually figure out where you are even if you don't have a GPS signal. Try driving in the SMART tunnel or under the numerous flyovers in Singapore where the GPS signal is weak and you'll realize how useful this is.
Just my experience using both systems.