while I'm sure I'm a
little biased, my vote also goes for the E90. I know Europe and her streets well and I can't imagine have anything bigger than a 3. If you don't necessarily have a need for a larger sedan, either, then I say E90
besides, it's not a step down. The 520i is lower in the 5 range, while the 330i is at the top of the 3 range (with respect to sedans, of course)