I'd go for the 330d personally. The book MPG figures for the 320d's are way off what they actually achieve. I know someone who's had one from new, kept it well serviced etc etc and still only manages circa 40 MPG at the very best. He's had it in at BMW to get it checked, but nothing was found wrong with it.

On the other hand, I believe the 330d's are more impressive - the milage gap between the two shouldn't really make a difference if its been well maintained. BMW's last forever - I get the use of one with other 210K on the clock, and its still going strong.