You need to drive both, ideally back-to-back.
The E90 is technologically superior--that's an objective fact. It has a more capable suspension layout and a more advanced engine design (more power, flatter torque delivery curve, slightly superior efficiency). The chassis will better protect the vehicle's occupants in a collision. These are incremental improvements, but improvements nevertheless.
However, the E90 is also another very big step down BMW's road to Buickdom. The traditional BMW DNA is still evident, but the overall experience is undeniably muted. The 330i with ZHP has a substantially more aggressive feel--it has a fast steering rack, and it's lighter, louder, and very nearly as fast as the E90 in a straight line.
Which will be more reliable? Who knows. By the time a car is 5 or 6 years old, previous maintenance and use are just as significant as anything else, so it varies from car to car. The E90 is still too new for all of the particularly nasty problems to show themselves; all of the common problems on the E46 chassis are well-documented. The E46 will be easier to work on (it's a simpler machine). Insurance and fuel should be a wash.
No significant quantities of judgment-impairing narcotics were consumed during or before the creation of this message.