Cast iron tubs are ridiculously heavy. So unless it's a priceless collector's item, you don't want to have any part of moving one. Your best bet is to bust it up in place. It can be tough to get the cracking started, so begin hitting the tub at the edge. Once it does start breaking, pound your way along the smashed edges.
Throw a tarp or thick sheet of plastic over the top to cut down on flying shards of iron. And wear your safety gear, especially hearing protection—busting up a cast iron tub is like having a front row seat at a church bell convention. This tip works on just about anything brittle: old toilets, radiators, concrete laundry tubs, etc.