I'm going to have to stick to the "it has fenders" reason, it's the only the one that makes sense to me. Emotionally, I will pick the more interesting bike, not a belly button (everyone has one) bike like the Cinelli. There a LOT of those out there, maybe not in as good condition, 1984 was not Cinelli's peak for quality but possibly quantity. That Legnano was from a time when Bartali was winning the Tour on a bike like that, what's not to love about that?

As for functionality, I would say the Cinelli is incrementally better over the Legnano as the latest carbon wonder is incrementally better over the Cinelli. That is a different discussion in a different thread on this board. If you have a need for speed, get a new bike. The 1984 Cinelli is probably like any other Italian steel from the 80s, why not try something different?