I owned a 323i E90, then bought my 320d E90. I loved that sound track of the 323i, once it was going it was awesome, effortless overtaking in 6th gear, fuel consumption on the open road was around 8l/100km and in town about 10.4l/100km. My 320d around town averages between 5.5 to 6.5l/100km, depending on how I drive, open road I get around 5.1l/100km at 140km/h.
That said, if I had top choose again in te range from 320i and 325i, I would take a 320d every time. Great car, sufficient power and awesome economy.