This is almost the same question I asked when I was deciding between the F30 328i vs. 335i.
After a test drive, there was no question. 3.0L I6 was the way to go. It accelerated way more effortlessly than the 2.0.
Fuel savings: 2mpg for 28i. (IMO, not significant as most of my driving is highway and there's only a 1mpg difference on the highway)
https://www.fueleconomy.gov/feg/Find...36142&id=36141
Vehicle weights:
335i auto - ~3695lbs
X3 28i auto - ~4100lbs
X3 35i auto - ~4230lbs
I've had a passenger + 500lbs of weights (used for alignment) in my 335i before. It was substantially slower carrying that load. An X3 basically starts at that weight. I would definitely want an 3.0 I6.