Is the Xdrive capable of torque vectoring to each wheel independantly like the Audi quattro sport set-up? When it is, there is no doubt: go for Xdrive all the way, including driving fun and sheer vehicle dynamics.
When I wrote "foolproof" and "relaxing", I meant that you can drive the holy smoke out of it, while it remains loyal to the traject you steer it in to. This under conditions where the RWD version has its' rear axle struggling for grip and you're on the verge of overpower, throwing the rear out, requiring instant counter lock to save your car.
There is no evidence that the Xdrive is more understeer, I believe that the center of gravity is moving a bit lower and a bit backward. Good conditions for even better dynamics!
Of course, +75kg is never good a thing, but maybe this is more than just compensated with the good elements that Xdrive brings.