It makes little difference. Fluoride is also toxic, and solidifies if you shut down the reactor. Try reading about the Soviet Alfa class submarines to know why putting reactors like that on vessels is a bad idea.PWR are pressurized. Aka, they can explode if when there's any issues and you need large concrete structures to contain the reactor. A molten salt reactor should in theory be smaller, lighter and safer. Also this is likely using fluoride, not sodium. Sodium reactors are classified under liquid metal reactors. Pure sodium isn't a salt.