Driven them both? If not, do so and see which one stirs your senses more. Honestly they are both fantastic cars and nit-picking between the two is just stupid. The true test is when you drive it, do you love the experience? Do you look forward to the experience? Simply put, which one stirs your emotions the most. That is the car you want to pick.
I think you can tell I am a BMW guy from saying that, but I swear to you it is the absolute truth. Even if it is a G37, Audi, or hell even a Honda, pick which one you love! The rest will fall into place. Oh and FYI, you can get a 3 series without iDrive. Personally, I prefer it without, however if you must have nav, the latest iDrive is WORLDS better than the previous generation and a breeze to use.