Ana's a lot more challenging, though her toolkit is a lot stronger if you're skilled with it. I definitely do more healing with Ana. I actually wish Mercy was a bit better, I like her aesthetic much more, but I find her leash to be terribly limiting. I've been playing a LOT of Ana lately, she's become my
go-to healer.
I had a really, REALLY satisfying game when I got done with work today: Joined a KotH in progress in Lijiang Tower (the horrible one with the bridges), saw the team was didn't have the point and had only one Zenyatta healing, picked Ana, and proceeded to heal my squad to victory. Mid-match replacements are usually such a dog's breakfast, it's refreshing to be able to turn one around.
Now my wishlist for a new Hero is this: I want a support ARCHER. I love playing Hanzo when I'm not support, because I love the arrow mechanics in Overwatch, and I would LOVE bringing that to a support role.