Bike size advice Bianchi vs Cannondale
When you buy a complete bicycle, you just need to check two things: fitting and components.

Did you test the 2 bikes in the size you are discussing? The Bianchi should have a better frame, but nothing that can counterbalance the fact that a 50cm frame is too big.

Regarding "If I go for a 50cm frame I need to cut 2cm in the seatpost (seatpin)" I would go as far as to say that you are voiding warranty and therefore it does not make sense to do so on a new bike. In my opinion, neither on a used one.