I compared the "trip computer" stats on the X3 and 335. Both trips were to the same destination (42 miles in morning traffic). The 335 has better MPG by 4 miles:
2011 F25 X3 (N52 engine, 6-cyl NA)
departure = 8:03 am
duration = 1:06 hr
distance = 42 mi
consumption = 25.1 mpg
speed = 39 mph
2012 F30 335i (N55 engine, 6-cyl turbo)
departure = 8:06 am
duration = 1:06 hr
distance = 42 mi
consumption = 29.3 mpg
speed = 39.3 mph
This is where I got the info. Next week, I will try the ECO mode on the 335.