Hello!
I've just reserve my ML, with telematic system.
I have to choose a Mercedes-Benz Bluetooth telephone module. I've read that there are two models.
SAP and HFP.
Where are you from? The BT SAP device should not be available for US cars (don't know if it even worked with latest cars).
I would say BT SAP is better because it uses the car antenna and does not consume the phone battery significantly (since the phone is not transmitting other than the BT connection to the car). BT SAP supports text messages to/from the cluster too.
The problem with BT SAP is that not all phone brands support BT SAP. Mainly Nokia and HTC I believe so far.