Which car to keep RX 350 vs E300

I would keep the Lexus. Generally speaking, your maintenance costs will be cheaper in the long haul. And, you'll likely find it to be more versatile.

Obviously an SUV is more versatile than a sedan, and the Lexus also has AWD. You can add an nice in dash OEM looking aftermarket GPS navigation system with tons of user friendly features for under $500.00 USD. That being said the E-class Mercedes' are generally made to last longer, as are most Mercedes vehicles, as can be attested to in their use as Taxi's worldwide. The Lexus will have expensive maintenance items such as Timing belt changes, that are NOT required on the Mercedes.

I personally strongly advise to keep the Lexus, first for the longevity on the vehicle and lot less expensive to maintain. I have seen those rx350 going going over 250k Not as much for the Mercedes. Good luck