IMHO, the 328 is the last and most developed of one of Ferrari's great model lines stretching back to the 308 in 1975. The cars lines flow smoothly and it is well on its way to being a classic. Even by todays standards, it performance is competitive (of the 3x8 models, my two choices would be either a very early fiberglass 308 GTB or a very late 328 GTB).
The 348 on the other hand, did not last very long, had a lot of negative press, went through to many model upgrades /changes, was not really a new design aesthetically, and suffers from beig sandwiched inbetween two much more highly regarded models. That having been said, it is still a very good drivers car.