I just got a 330Ci this past weekend so I'll share what my thought process was.
I went for the 330Ci over the 325Ci because for the supposed same fuel consumption (20city/29hwy) I get a larger engine with more torque and power. I personally don't get why that is, the fuel consumption being the SAME, but it bothered me. I also wanted the Harmon Kardon audio system that comes standard w/ the 330 for some odd reason and I really, really wanted the M Double Spoke 68s that come with the 330 Sport Pkg. The idea of the wider 245s in the rear with the "narrower" 225s up front really tempted me. I like the look of the 325's Sport Pkg. wheels but they are the same all around plus don't look as uniquely BMW as the Style 68Ms.
This might not be the case for you but alot of 325 owners I know personally regret not throwing in the extra $3-4k for the 330 later on so I went with their recommendation. 325s are also fairly abundant on dealership lots right now. When I went to a dealership and said I was looking for a 330 they tried fairly hard to turn me to one of the many 325s they had on the lot (they had no 330s. No coupes, no sedans). Turned me off to the 325 even more.
Oh, I bought new btw. I have no idea what the market is like for used cars.