1. Don't particularly care for turbos & feel that BMW's non-turbo 3 liter 6 is one of the best engines around.
2. More reliable motor.
3. Overall balance of power & efficiency - good gas milage with acceptable performance which rewards skillful driving.
4. 165 lbs. less weight.
5. Lower insurance costs.
6. Had xenons on a Z4 & frankly, since I don't think they are worth the money (especially if repairs are ever needed), I preferred the option not to have them. YMMV.
7. If I need an acceleration rush, my BMW motorcycle will run rings around a 135i.
(Ideally, I would have preferred an 130i with around 260 bhp.)