I have recently acquired a 2017 DT350i with less than 5000km on the clock. After 80,000km with the DT300i and numerous issues that developed over the past few months it was time to move on.
First impressions: The 350i carries over nearly everything from the 300i that made it good. The engine looks and feels exactly the same, the transmission is a bit smoother and the controls are where you expect them to be. The seat is exactly the same which is good for me, a short person. Even the service intervals are the same.
Pros: improved headlights, front LED indicators, LED taillight, better access to the battery and rear brake pads, seat opens easily
Cons: blue dash is a bit too bright, both gloveboxes feel flimsy, key cover is harder to open due to angle
Is it worth upgrading from a 300i to a 350? Probably not as there is not much difference. However if you want to a new scooter then the 350i should be at the top of your list.