I think the differences between the V12 and the V8s are too far apart for comparison. Between the 348 and the 355, the 355 is easier to live with. The 355 is a much sweeter car and a joy to own. The maintainance cost is the same so might as well live with something that's nice.
The 456 series 1, can be quite problematic. The window won't close properly and you'll be annoyed because ther is nothing that can be done. But if you can lay your hands on the 456M then plonk for that car. It's more expensive yes but all the problems of the original 456 has been solved with the M model.
Hope this helps.