I think it's almost pointless to compare iDrive vs MMI vs. whatever Mercedes has. You're going to have people who like one over the other, some will find one easier to use while others difficult, almost like a Mac vs. Windows debate.
As for the layout, I'll take BMW simply because everything is easily laid out and it's really where everything should be (traditionally speaking). Radio, climate control? On the dash/HU. iDrive control? Close but away from the shifter but still easily accessible. Everything is separate/clearly laid out and not clumped together like the Audi.