Depends of your needs and preference, as always.
People talk a lot about Alistar W-Q combo that's like a mini malph ult. But he has the possibility to peel harder than janna while being tankier than any other support. Janna has more burst peel, but with enough cdr, Ali can rotate all your cc to prevent whoever you want, no matter how fed they are, to go past your frontline. Awesome VS stuff like Riven, Darius, fat tanks you want to keep far from your squishies.
Leo has... more early tankiness, an easier time engaging (more range, more reliability), more burst lockdown especially with cdr (Q-E-R-Q combo works for 4 seconds of deny, beat that!). And her ult has a really low cd, allowing her to blow flashs and try again several times. She can still peel, but doesn't have the displacement of Alistar, and tanky targets can buy tenacity against her. Awesome vs marksmen, mages, a little less vs squishy damage dealers.
Can't talk about bronze-relevance, they both have to time their abilities to max their impact and don't do bad decisions (going in when it's a dumb idea, or for alistar, knocking back someone you could kill).