I should imagine if your looking second hand the 320 would be the pick.
You would probably wish you had, and they tend to have higer spec.
Funny enough when I was looking at 320 sports a few months ago, most estates at least were 280's. Was it better for tax reasons?
I know a few years ago I was after a 3 series convertible 325 or 330, I held out for the 330 not because I need the power...just because I would always wonder if I should have waited.
As above performance / mpg and just about everything else probably very similar.