BMW RaceR, when you want effortless fast driving, I'd still go for the Xdrive.
When you drive the Xdrive, I'am certain you'll confirm it is much more foolproof and relaxing. Of course, not everybody associates an M135i with relaxing. But I guess that in +90% of all driving conditions, this prevails and determines the daily experience with the car.
Take note, it weights more, but the weight is favourable located: under the center of gravity. So it is not certain that it negatively impacts the composure/dynamics of the car during fast driving.
Besides, price difference between RWD+AT and Xdrive+AT is really ridiculous small, just like fuell efficiency impact.