Although I'am not directly in ordering one of both (Although I both want them...) I would use this sceme:
1) do I have at least 2 children (coming up soon) and this is my first and only car? Yes --> 328i, No --> M135i (When it is your second car, it doesn't matter, assure your first one is big enough
)
2) do you drive more than 15Kmiles/year mainly on high roads with cruise control? Yes --> convert to 328i, no, keep M135i
in case still on M135i, do you have to look distinctive to 3rd parties often (a few high class parties every month where you have to "shine" for people age 50 and more?), Yes --> convert to 328i, else keep M135i
I hope 75% lands on M135i, what a splendid machine it will be!