I was given a new F30 320i xdrive M Sport auto (on 19s) as a courtesy car last time my car was in the dealership (so fair amount lighter than a 528i xdrive). Initial impressions (in comparison to the m5) were much smoother low speed ride and fantastic traction/handling, effortless incisive steering, no less refined than the M5 CP (although less than a std. F10). Had me thinking how good a 335d xdrive would be...
...until I got to drive at higher speeds and the ride was appalling - I could not believe how much I was being jostled by the car & somewhat perplexed at why the ride fell apart in the manner it did at higher speeds. Got home & checked the tyre pressures - 39-40 PSI all around (over inflated at the front).
I changed them all to 2 PSI above the minimum setting (on cold tyres the next day) and set off again. This time, although the high speed ride was no longer unsettled, the car felt slightly worse than the M5 CP (which I run at 1 PSI above the minimum tyre pressure setting) - handling not as sharp as before. As said above, lacked that 'soul' that M cars have, i.e. it didn't feel greater than the sum of its parts. Xdrive didn't seem to be adding anything to the handling at normal public road speeds (at least in comparison to the M5 & in warm, dry weather).
Now that the weather has improved in the UK, I have also had a 'handling epiphany' in the M5 CP - it is actually very capable on tight twisty B-roads (technical roads), in fact, I have to admit it's the first time I've thought the handling was exceptional - my old adaptive drive equipped RWD F10 535d M Sport would have been left for dead on the same roads. Unfortunately 6 months of the year in UK it's too wet, cold, otherwise slippery for the PSS tyres to work as well they are currently. On that basis, I would be tempted to tick the xdrive option if it appears on the next gen, if only to cover the 6 'bad' months for M5 driving in the UK. If, however, I lived somewhere where the weather was better, I'd stick with the RWD.