I'd probably recommend Leona. She just seems much more consistent. Blitz is just so weird with his hooks. One good hook can decide a lane, and even a game. Other times, you won't be able to hit one good grab. I don't like how everything is based on a grab. Leona isn't as potentially game-changing, but she's a pretty good support tank, offering a good amount of cc and damage.
If you're planning on buying a new support, then don't get Nautilus. His status as close to perma-ban will not let you play him much. If you already have him though, I'd put him as higher than Leona. His cc is a bit stronger/more reliable, and he's a better tank early on. He also has an escape in the form of his Q, something Leona doesn't have (when she goes in, she's all in). Not saying he's a better support than he a jungler, but like Alistar, he still performs really well.