Mardi Gras is Carnival's flagship. I would much rather go to Half Moon Cay than Nassau however and I don't think anyone would disagree with that. So it really depends, are you more of a ship cruiser or a destination cruiser. If the ship is your destination, then Mardi Gras. If you're more interested in the ports of call, then Vista.
Frankly although I love Half Moon Cay I don't hate Nassau and I am more of a ship as the destination cruiser anyway so I'd pick Mardi Gras.