I think if you will be doing a lot of city driving, then the 4 banger might be preferable. The 6 banger is good if you do longer drives and on highways. You get the chance to floor the engine. That's the only time you get to enjoy the sound of the engine.
I drive 25 KM to work every day and right after I get out of the Western tunnel, I just floor it and enjoy the sound
Of course, there's that other thing about matching the size of the car with the engine. On a 5er, I prefer a bigger engine. On a 3er, I'll be very happy with the 4 banger. I think this is more of a psychological thing.
Best way to know which one you prefer, do a test drive.