From what I understand, the 335D will have more issues then the OM651 GLK250. Think headgasket and other related problems, but I have not done any research just hints here and there and not much resale once out of warranty for the BMW. The GLK will generally be easier to service more space to access components.
There is special oil for those 4 cylinder diesel MB229.52 now, I have not found this printed on any consumer jug of oil, probably dealer only for a bit yet.
I took one out for a test drive when they were released, so nice to drive on rough roads and plenty of power for what it is, wish all of them had the 4 cyl instead of the 3.0 V6. Unfortunately they didn't want to offer anything for my R320 on trade or it would have been an easy sale. Everything is complex these days so don't worry too much about it, run the best diesel fuel you can (research, you want 50 cetane and a busy place with lots of volume, avoid anything over 5% bio diesel).