Lead is heavier, doesn't rust, and will deform a bit if you smack into something. A friend of mine hit an underwater rock with his c36 at around 6kts which made a 4" gouge in the keel. No other damage (except him flying across the cockpit) to the boat so it was a simple re-fair that winter. That same hit to a harder iron keel - may have - done more damage to the keel connections and the people on board.
Wickford/Narragansett Bay RI