Iv a 320d and a family member has one year newer 318d. This family member drove ours one day and said he couldnt believe the power in mine. My husband then drove his and our was much more powerful in having power to take off. The bhp is different, buy the 320d for the extra power and torque.
they is €1-€2K differ in the same year between the 320d and 318d. The dealerships have a glut of 318d at the moment as we looking for a 320d all summer, they'll tell you no difference if they only 318d in stock. There attitude sours immediately when one points they is, that you'v driven both.
(the only issue Iv with my 320d is the back seat doesnt go down, funny in previous cars Iv had you may use the back seat down once/twice a year but when you cant do it you'd miss it like when buying a large canvas, kids new boxed bike etc)....