For my money, mold temperature is the critical part when casting big bullets. With a hot mold and hot alloy, I get fine results with both bottom pour and ladle. With lower temperatures, neither works especially well.
Having said that, I generally get more uniform weights when I use the ladle, especially with 500+ grain bullets. That matters quite a bit in some applications - long-range blackpowder target guns, for instance - but I'm not sure it would make any difference at all for things like shotgun slugs.