Get both? Lol.
Honestly, 355 does it for me. I love the testarossa; but power nothing (steering, windows?, seats) means u'll be having a tough time living with it daily. I know you probably won't drive it daily anyway, but believe me, driving a non-power assisted car isn't a hoot. I drove a 348 a few days ago, and i was shocked at the weight. I'm sure others would (especially 348 owners) disagree with me here, but good god, it was really heavy!
I guess in the future, as technology infuses cars with more driver assistance (F1 gears, Electronic Damping, Servo Assisted Brakes, HUD, Launch Control, etc..) we'll soon be scratching our heads on how we actually worked the clutch in first place!
I hope that doesn't happen, cause nothing beats a proper manual when there's a clear road.