My thoughts.....keep the 335d and keep racking up miles. You'll take a bath if you trade it in. Besides....a Jetta may be nice...but it is not even the same ballpark as BMW nice. Why take a bath for a step down?
Worst case....if the 335d starts acting up down the road, then your hand is forced to trade it in and take a bath. But for now, with a good running car...I say keep it and keep racking up miles on it.