Wow! You know what you're doing when you buff out a car.
Can I just give you my M235i and let you give it a makeover for me?
All things aside, you're considering 2 cars with similar specs. In a nutshell, if this is going to be a daily driver, I would take the M235i hand's down. Better city ride, back seat, usable trunk, better MPG, easier to climb out of, etc. Plus as others have said here (and most car reviewers on Earth) the M235i is a special car.
That said, if you've already got a solid daily driver and want a pure weekender/canyon cruiser/roadster, then the Z4 wold be pretty tight. Just make sure you won't ever need seats for 4 people or a large trunk.
Good luck!