The I6 will always be smoother; also in terms of how it delivers power. The six cylinders are going away for all BMWs in the future, so you'll have to get used to the I4 turbo.
It's about the gearing, because if you read the F30 magazine tests; the majority stated go for the 328i. The new N20 engine has already been awarded engine of the year.
Note: you are comparing x to x, since both models are 4WD. The 28i comes in a RWD version which means less weight. You might want to consider that as well.
Note 2: a lot of people are finding that the price difference is around 2-4k between both models. (depending on the packages) So you will have to build two out and see.
Note 3: just test drive both.
