As an original owner of a 1M and an owner of a '99 355 berlinetta, I can offer a direct comparison. I love my 1M. I drive it every day and it is incredibly fast and understated. I get an occasional thumbs up from a BMW guy but most people see it as a little BMW and I like the stealth. It is comfortable to drive and is incredibly affordable to service (free in the USA -- don't know about the northern cousins). It makes great noises and can push the right buttons when pressed.
The 355 on the other hand is a little slower and less comfortable to get into and out of. The doors are surpisingly huge and require a large space to open -- not convenient for a daily driver in the city where parking can be a problem. Further, if you don't want people looking at you then the Ferrari is not the answer. Mine is 13 years old and old women, young boys and everyone in between stares at the car when it goes by. It is about the most incredible sensation to hear and feel that car open up. It is incredibly stimulating in ways the BMW isn't and is surprisingly comfortable on long drives. The biggest downside is the cost of ownership. I have a great mechanic who is very reasonable, but major services are indeed major and I think you can reasonably expect to spend $5k/year in service on the Ferrari if you take care of it and want it to be right for the long run. If you know what to expect, then I think it isn't a shock.
I love both of my cars and have the best of both worlds. I really can't recommend one over the other, but there are plusses and minuses to each. If you have any other questions, don't hesitate.