The difference is not much. I remember when i was buying my 335i, i also looked into the difference (was under 50 dollars i believe...dont remember the exact amount).
Dont quote me on this..but my uncle who worked in insurance explained to me how the 3 series falls more into the luxury category as opposed to a nissan 350z which falls into the "sports car" category. Also bmw's are cheaper insurance wise compared to the 350z because of their safety ratings.
Either way...as others have said, it depends on the individual.
Go for the 335i!
