I would say Zyra is the better support, but Leona is easier to play and, has (IMO most the time) a strong laning phase.
Even if the Leona isn't going in all the time, the threat of it happening is enough for the other side to either get surprised or get zoned.
It does depend on the person and etc, but I just feel like if Leona doesn't help their adc/team snowball, she becomes less useful lategame than a Zyra late game, though, this is ALL speculation, I'ver never met a good Zyra, considering I'm bronze-worthy and only play normals.
Renekton is where it's at [slayer-pantheon-thumbs]