Everybody has there own personal tastes and opinions, so a lot of it comes down to what you particularly want from the car. What sort of driving do you do? How many kms per week? Are you all about fuel costs, driving style, engine noise, etc?
I have done considerable kms in both models as both F30 and F31 wagons. As I tow a fair bit, the diesel was a better proposition for me, the low down torque and better fuel consumption (especially comes to the fore when towing).
As others have said, and you have noticed, the diesel has great low down pickup, whereas the 328i you have to rev more to get the best out of it (which doesn't help achieve the book fuel consumption).
Anything to look for, hmm, would be the same between both, no real differences by model. Look for the bells and whistles - professional navigation, head up display, etc. Might not find a Jap import with all the options, and you will find more 328i Jappers than the 320d I would suggest. For me Motorsport versions look a lot better than the other "lines" but Sport line is still quite good.
Whereabouts in the engine bay is the yellow sticky stuff? Without a photo my first quesss would be the spray wax that is used to seal and lubrucate certain areas, compare it with the stuff on the door hinges - if it's wax it will look the same.
Drive both, there is not a lot between them, and Qube says it might be a case of finding the right colour combo and spec and going with that rather than the engine.